Many thanks to tball for the inspiration that led to today's fanpost.  This isn't the complete comparison of the A's and Rangers farm systems that tball suggested to Jamey, but it was a fun dataset to try to get my arms around.

A lot has been made of the recent accumulation and development of pitching prospects by the A's and Rangers.  To get a sense of how the A's and Rangers stockpiles of young pitching prospects compare, I gathered the 2008 stats for the Top20 or so pitchers under the age of 27 for both teams and categorized them as follows:  AAA/ML, A+/AA, A-/A, and Rookie.  Categorizing the pitchers made it easier to rank the pitchers from both teams since it eliminated the "ceiling vs probability of reaching ceiling" issue that makes stack-ranking younger and older prospects together so difficult.  Given that I know more about the Rangers prospects, the analysis was undoubtedly biased in the Rangers favor; for that reason I would highly recommend that you spend a few minutes with the stats below and draw your own conclusions.  The column entitled "Level" provides the level(s) that I used to compile the data that is shown.  "BAA" is Batting Average Against, LD% is the percentage of balls in play that are line drives, and FIP is a stat that combines the rates of home runs, walks, and strikeouts to estimate ERA.  In theory, the stat eliminates the influence of defense though its elimination of balls in play tends to under-estimate the performance of groundball pitchers.  The FIP used for the table below featured a park effect calculation to minimize the potential effect of a home-field advantage or disadvantage though the normalizer does not account for differences between leagues.   

AAA/ML
                   Age/Hand Level  IP    ERA   BAA   WHIP  BB,K,HR/9IP LD%  FIP
Smith           24/LHP    ML   160   4.23   247   1.37     4.2,5.6,1.0
Eveland       24/LHP    ML   139   4.21   260   1.47     4.4,6.1,0.5  
Gallagher   22/RHP    ML     93   5.23   274   1.59     4.7,8.0,1.0
Gonzalez     22/LHP   AAA   126  4.24   237   1.33     4.4,9.1,0.9     16      4.40
Braden        25/LHP   AAA     54   2.36   256   1.15     1.8,9.0,1.3     21      4.20
Meyer          27/RHP   AAA   126  4.48   251    1.33     3.7,7.8,0.8     15      4.28

vs

McCarthy   25/RHP      ML     16    2.25  228   1.13     2.8,5.1,1.1 
Feldman    25/RHP     ML    133   5.05  278   1.44     3.6,4.4,1.3
Harrison     22/LHP    AAA     88   3.43   271   1.33     2.9,5.7,0.6     13      4.13
Hurley         22/RHP    AAA     87   4.83   279   1.40     3.1,7.7,1.8     18      5.42
Hunter        22/RHP  A/AAA  166   3.41   270   1.23    1.8,5.7,1.0      16     4.42
Nippert       27/RHP    AAA     64   3.98   275   1.28     2.3,6.1,1.3      18     4.89

There are no stars among either team's ML-ready pitchers, but there are a lot of solid pitchers who can fill out a rotation and bullpen. Oakland's ML-ready pitchers are almost universally left-handed and league average.  Eveland and Smith have proven to be effective despite relatively high walk rates.  Gallagher is producing about as well as could be expected for a 22 yo pitching his first season in the major leagues.  Gonzalez generates a lot of K's with a plus curveball but his average fastball and change-up have made his transition to the big leagues difficult (7.18 ERA, 1.78 WHIP in 26.1 IP).  McCarthy's injury this year makes it difficult to compare his stats to the other pitchers, but as recently as 1.5 years ago he was considered to have perhaps the highest ceiling of any player on either team's list.  Feldman's age and peripherals peg him as being essentially equivalent to Smith and Eveland.  Harrison and Hurley have both shown that they can pitch at the big league level, though they are currently below average with mid 5 ERAs and ~1.5 WHIPs. During the next few years, it is likely that Hurley, Harrison, Hunter, Gallagher and Gonzalez will be performing at similar levels in the big leagues.  Nippert was thrown in as the sixth Ranger, though Gabbard or Mathis would have fit in just as well.  Given McCarthy's questionable health, the AAA/ML category falls on the A's side of the ledger.   

A+/AA
                    Age/Hand   Level      IP    ERA    BAA   WHIP    BB,K,HR/9IP    LD%   FIP
Cahill           20/RHP     A+/AA   128   2.61    185    1.01        3.5,9.5,0.4       10      3.26
Anderson    20/LHP      A+/AA   108   3.69    242    1.15        2.3,9.8,0.7       13      3.51
Simmons    21/RHP        AA      137   3.51    279    1.32        2.1,7.9,0.8       23      3.53
Mazarro        21/RHP   AA/AAA   173   2.74    249    1.15        2.3,6.5,0.3       21      3.30
Rodriguez    21/RHP       AA        43    5.20    298    2.23        9.3,9.1,0.2       21      4.80

vs

Feliz             20/RHP     A/AA     129    2.69     204   1.10        3.6,10.7,0.1    18     2.59
Holland       21/LHP      A/AA    154     2.27     210   0.99        2.4,9.4,0.2      15     2.63
Kiker            20/LHP        A+      124    4.73     292   1.42        2.7,8.0,1.0      12     4.55
Poveda       20/RHP        A+        94    4.47     241   1.31        3.8,9.3,1.0        8      4.58
Hyatt            24/RHP    A+/AA     143   3.58     239   1.23        3.0,7.1,0.8      13     4.18

The Top2 pitching prospects for both systems finished their seasons in AA.  The following question has been asked of national prospect experts, "Would you rather have Cahill and Anderson or Feliz and Holland?"  To date, the answer has been Cahill and Anderson, but Holland's amazing finish might be changing the minds of those who are in the know.  The remaining prospects in the A+/AA category are also very interesting.  Simmons has outstanding control but gives up a few more hits than you would like and doesn't strike out enough players to be considered a top-of-the-rotation pitcher.  Mazzaro pitched great in AA (1.90 ERA, 1.10 WHIP) but has struggled in his first 33.2 innings in AAA (6.15 ERA, 1.72 WHIP).  Henry Rodriguez has an upper-90's fastball that he struggles to control.  He struck out 147 batters in 117 innings, but also issued 84 walks.  His transition from A+ to AA has not been pleasant (3.96 vs 7.46 ERA).  Kiker and Poveda both had good, though somewhat disappointing seasons in the Cal League.  Both pitchers use three pitches to produce almost 1K/IP.  Though Poveda posted a slightly higher walk rate this year than in the past (3.83/9IP), both pitchers are considered to have good control.  With the exception of the ERAs, which are difficult to compare between the Cal and Texas leagues, Kiker's and Poveda's numbers compare favorably to those of Simmons and perhaps slightly above Mazzaro due to Mazzaro's low k-rate. The fifth pitcher on the Rangers list posted outstanding numbers in stops at A+ and AA (and a spot start in AAA), though his age and lack of outstanding stuff leave him with lower upside than Rodriguez and tips the scale for the A+/AA pitchers in Oakland's favor.        

A-/A
                   Age/Hand   Level      IP      ERA    BAA    WHIP    BB,K,HR/9IP    LD%    FIP
Santos         22/RHP    A('07)  122      2.65                0.90          11K/9IP
Hernandez  21/LHP      A-/A      76      2.04    222     1.03        1.7,7.9,0.1      14       2.73
Ross            21/RHP       A         17      4.66    206     1.06        2.7,7.4,0.5      10       3.53
Italiano        22/RHP      A/A+    103     3.81    223     1.40        5.3,9.7,0.7      17       4.36

vs

Main            19/RHP        A          46     2.58    228     1.10        2.7,9.8,0.8       17       3.34
Beavan        19/RHP       A        125     2.37    233     1.01        1.4,5.3,0.9       14       4.19
Boscan        18/RHP      A-         71     3.12     251     1.10        1.4,8.9,0.4       15       3.04
Ramirez       19/RHP     A-          45     2.66    163     1.20         5.8,10.4,0.8     15      4.78
Perez            17/RHP     A-          65     3.65    276     1.47         3.9,7.4,0.3       19      4.06
Gomez         20/RHP      A           90     2.97    235     1.08         2.0,7.0,0.8       16      3.80
Murphy         21/LHP    A-/A         48     2.83                1.00         3.4,9.6,1.0
Pimental     18/RHP      A-          65     3.31                 1.21        4.3,7.4,0.6

The biggest disparity between the A's and Rangers minor league systems occurs at the low A levels.  Fautino de los Santos put up very good numbers in the Appy and SAL leagues last year (2.65 ERA, 10+K/9IP, 0.90 WHIP) but he has missed all but the first month of this season due to an arm injury.  Carlos Hernandez is a 21 yo LHP who posted very nice numbers in 75 innings of work in the NWL and MWL, however he is not included on any list of A's prospects that I've seen.  As a 2nd round pick in the 2008 draft, Tyson Ross has not pitched enough to know how he will transition to professional ball.  22 yo Craig Italiano opened the year with some of the best stats in the minor leagues as he repeated the Midwest League after an uninspiring 2007.  His early season 1.16 ERA, 10+K/9IP, and 1.11 WHIP is now looking like a mirage as he has put up a 9.90 ERA and 2.33 WHIP in 30 Cal League innings.  In contrast to the A's, the Rangers low A minor league rotations are absolutely stacked.  No fewer than 5 pitchers are prjected to be mid-rotation or higher starters when they reach the big leagues.  Main, Ramirez, and Perez are all considered to have top-of-the-rotation stuff and posted numbers that support that contention.  Boscan combined high k and groundball rates with a low walk rate to dominate the NWL as an 18 yo.  Beavan and Gomez were among the MWL's most productive pitchers despite rather pedestrian k-rates.  Throw in Tim Murphy and Carlos Pimental and it is hard to imagine this wave of pitchers failing to produce fewer than 3 major league starters.  This category is an absolute slam dunk in favor of the Rangers.

Rookie
Inoa

vs

Font, Weiland, O'Campo, Ross

This is where my lack of knowledge of the A's farm teams and recent signees makes it difficult for me to compare the two systems.  A review of the A's rookie league team failed to reveal even a single pitcher with a compelling stat line.  Micheal Inoa is being touted as the next Felix Hernandez, but beyond that it appears that the A's low minor league cupboard appears to be bare.  In contrast, Weiland and O'Campo were among the AZL league's leading pitchers, Robbie Ross is touted to have three solid pitches, and Font is at least in the same neighborhood as Inoa in terms of potential.  As with the previous category, the collection of Rookie-level pitching talent clearly favors the Rangers.

Overall - Oakland might have a slight advantage at the upper levels, but the depth of high ceiling arms at the lower levels provides Texas with a clear advantage in the AL West Arms Race.