9mm's a lot cheaper to shoot, 40's a little more power and more recoil. Depending what ammo you use, the difference between the two is marginal according to comparison tests I've seen. I'm sure there are those who'll disagree, but if it's me, I go with the 9.
Donn, ammo is no problem, I have 5000 rounds between the both that I got at a great deal but like I said, I am not buying them to shoot but to sell in the future for a profit if possible. I am a proud glock fanboy and I don't see that changing.