well, i am going against the grain here and saying 9mm. I have the XD service model 9mm (what a fantastic pistol btw) and shoot it all the time. the reason i bought the 9mm is cheap ammo. you can get better at shooting it if you can afford to shoot it. Also, with the bullets available today you can find a great defense load in 9mm.

Walmart was selling boxes of WWB 9mm ammo for $7-8. the .40 was $14-15.