Well, it's only applicable to STI but they're the only ones nice enough to provide the numbers sfaik. This is from the Jan '09 Skinnergram:
The .40 edged out both the .45 and 9mm in double stack. Interesting.
Here's some gun stats for you. Single stack beat the doubles by 2%. (51%/49%)
By caliber:
Single Stacks:
9mm = 50%
.45 = 40%
.40 = 8%
.38 = 2%.
Double Stacks:
.40 = 37%
9mm = 33%
.45 = 20%
.38 = 9%.
The .40 edged out both the .45 and 9mm in double stack. Interesting.