9mm...with courses of fire the way they are these days I'd go for the capacity.
9mm would mean 'Minor' so let's see if yours is a common opinion.
At last year's Single Stack Nationals, barely 13.5 % (37 of 274 shooters) shot Minor.
http://www.uspsa.org/mr/stage_display.php?year=2010&match=2010_Single_Stack_Nationals&dir=Single_Stack&file=Pistol_Finals.dat
(and of those that show 'Minor' on the final results, some may have declared 'Major' but chrono'd 'Minor'. I don't have visibility on those stats)
Of the five GMs, all shot Major. Of the top ten finishers, only one shot Minor (B.J. Norris) and he came in sixth. Of the top five competitors shooting Minor, they came in 6th, 13th, 26th, 59th and 68th. So shooting 1911-9mm is hardly a 'winning setup'.
Of the 8 Area Matches conducted last year, the ratio of Single Stack competitors shooting Minor are similar (or worse):
Area 1 - 2 of 22 (9%) both of which were DQ'd
Area 2 - 7 of 43 (16%) with the top Minor shooter finishing 6th
Area 3 - 0 of 22
Area 4 - 0 of 13
Area 5 - 5 of 40 (12.5%) with the top Minor shooter finishing 20th.
Area 6 - 3 of 31 (9.6%) with the top Minor shooter finishing 5th. (Julie Golob)
Area 7 - 2 of 14 (14.3%) with the top Minor shooter finishing 2nd.
Area 8 -
Can't find the data.
I guess in Single Stack 'capacity' isn't all it's cracked up to be.