This was my big decision...9mm or .40? I ended up choosing the .40 for several reasons:
A) Recoil does not bother me. It does affect aiming for following shots, but all that does is prompt me to practice more than I would with a 9mm.
2) Simply put, it's bigger. If a .45 is bigger than a .40, a .40 is bigger than a 9mm. Even if it's just a little bit, it's still there. You could start with a .22, then say, "well a .25 is just a bit bigger." Then you consider a slightly bigger round, and keep going, well then you hit .45. In other words, a slight increase in bullet diameter counts for something. If it didn't, several increases put together wouldn't count for anything.
D) This is a pretty lame reason, but EVERYONE goes out and gets a 9mm so they can say, "yeah I got me a NINE." I am a non-conformist, so therefore I like having something different. Flame all you want for that one.
16) You can rebarrel for a smaller size, but not larger size. I could change to a 9mm configuration in the .40 if I ever felt the need, but you can't do it the other way around.
X19) I find it a lot easier to count in sets of eight rather than ten. It's kind of hard to explain, but I'm very musically inclined, and in most music you count in fours, as most measures have four counts. So since I was barely able to walk and surrounded by music, I learned to count in multiples of four. It's engraved in my skull. Yes, I can count to ten. But I prefer to count in multiples of four, and I like to keep track of how many rounds I have in the magazine. I don't like relying on the slide locking back to let me know I'm empty.
258) Not a reason for choosing the .40, but a reason for appreciating it. The front diagonal portion of the slide on the 9mm has an indentation before it reaches the end, where the .40 does not. I didn't notice it 'til after I got the .40, but personally I think the P99 is way more attractive with the slide diagonal staying flat all the way to the muzzle.