I carry a Glock 29 and practice with it often. I'm very, very happy with how well I can shoot this pistol. It holds 10+1.
I know for a fact that I cannot shoot it as quickly as a 9mm. That's the only concern I have. If I had to find a reason to carry a 9mm instead of my 10mm, it wouldn't be for more ammo -- it would be to be able to double or triple tap more quickly.
I think there is definitely something to be said for a certain capacity -- but that number is up to the guy carrying it. I did a handgun swap with a guy and received a very nice S&W Model 60, 5-shot J-frame .357 Magnum. He liked the revolver and carried it 100% of the time but he had a run-in with three younger (and motivated) guys and he felt undergunned with only 5 shots and three adversaries.
I met this man -- 6 foot 7 and 300+ pounds. When three guys came at HIM, I'm sure they meant business.
I've always thought that 5 or 6 shots from a revolver would be enough -- I am confident in my skills, I've never been the "spray and pray" type. But this guy's words are something I won't ever forget.