Go rent a .40 at your local range and shoot one first. I had a glock 23 40 cal, almost identical in size to P99. I learned to shoot on a 1911 .45 and I personally can't stand the 40. Every 40 cal gun that I have ever shot, with the exception of Beretta's Cougar, seem to torque/twist in your hand when you shoot them; at least that is my experience and that of several others I have spoken with. I would go with a 9mm myself. The best HP's in 9mm seem to do just as well as most of the 40 and 45 cal HP's out there. And b4 anyone tries to start a caliber argument I said that the best 9mm HP's do as well as "most" of the 40 and 45 hp's out there. The best 40 and 45 cal stuff still performs better, but not by much if you're good with your shootin iron and place your shots where they count. Shot placement is paramount. Get what is comfortable for you to shoot and practice, practice, practice!
I like my p99 9mm titanium so much that I will shortly be buying an all black one to keep it company.