Yes, 9mm is enough. Ammo technology has gotten a lot better. Also, you do some research and you will learn that bullets of any caliber are poor man stoppers. Shot placement is key. If you can be more accurate and/or shoot more rounds accurately with the 9mm, that is the way to go.

The ammo is also cheaper, which allows more practice.

Finally, the P99/PPQ platform in 40 is pretty snappy. I think you will like it in 9mm much more

I have the ppq in 40 and have shot it in 9. I think the only people saying its way more snappy in 40 are people that haven't shot it in 40. 40 with 180gr is no more, and I think less snappy then most of the small +p 9mm. But its hard to argue with a few more rounds and cheaper ammo. Just don't let "its snappy" be your deciding factor, unless you shoot it!

I carry .380....I can put each bullet exactly where I want it......I had a P99...nice, if you like plastic guns, I prefer steel, and a hammer. I traded the P99 for a .380 S&W. Like shipwreck said, any caliber is good, if they go where you want them to. I replaced 40 cal. with a Beretta 96, so I still get my .40 fix. Some people prefer 9mm, some .40....sometimes, that has to do with the cost of ammo, sometimes, the recoil.....I can be just as accurate with 9 or 40. I agree with hmott....+p 9mm is snappier than 40, in alot of cases.

Whatever you decide, SHOOT a bunch of choices first, before you buy.....it insures no buyer remorse.....

9 should be cheaper to shoot and from what I've seen of tests, more accurate than 40. Whether that's something inherent in 40 or it's because of more recoil I can't say.
As for more rounds, well, if you get yourself in a situation where you need 10, 15 or more rounds you've done something terribly wrong IMO - either your a bad shot or shouldn't have taken on a group of bad guys with guns!

NYC police did a report where they found the avg rounds expended in a gunfight were 4.

Why limit yourself? Get one of each!