My primary choice was 9mm because:
1. .380 not allowed at IDPA
2. 9mm ball ammo is less expensive than .380 or .45 (savings add up)
3. 9mm SD ballistics have improved significantly over the years
4. 9mm capacity typically exceeds pocket pistols and the 1911 platform
5. A majority of shooters have also chosen 9mm
In recent years, IDPA stats show that 9mm participation has increased, while .45 has declined. The top brand is Glock, however the M&P introduction has steadily gained on Glock.
Learn the proper grip for semi-auto (not the same as a revolver) then see what feels the best at your LGS toward finding the right gun for you.
If you're not inclined toward IDPA, a standard size 9mm polymer gun still has a lot of flexibility to enjoy. (I've also shot mine at Steel Challenge, Pin Matches, 2-gun matches)