I carry a Kahr PM40 routinely.
I don't particularly find it difficult or uncomfortable to fire, or to achieve reasonably rapid hits with.
In terms of capacity, its all relative.
If I can't resolve a sudden social dispute with 6 or 7 rounds of .40 S&W, it isn't clear to me how 7 or 8 rounds of 9mm is going to give me a significantly better chance. Given a spare magazine and the ability to reload with it, capacity seems to be one of those criteria that borders on irrelevant.
In terms of effectiveness, in all honesty I suspect that 2 or 3 solid hits with either 9mm or .40 S&W is going to remind an attacker that there is somewhere else he urgently needs to be. I'm unsure that its going to make a lot of difference - that said, in terms of ballistic energy there is measurably more energy with the .40; and assuming both projectiles expand, the .40 expands to a marginally greater diameter.
I suspect that if a shooter routinely works out with whichever caliber s/he prefers, that shooter is going to do as well with either caliber as the circumstances allow if they ever find themselves in a gunfight.
Just MHO. YMMV.
Treat everyone you meet with dignity and respect....but have a plan to kill them just in case.