I have a .40 and a 9mm........I just picked up ammo today for each.
Wally World winchester White box:

100 round .40 value pack = $21.99
100 round 9mm value pack = $14.98

I actually enjoy shooting the 9mm more, it's cheaper and trust me some of the hot defense loads in a 9mm are nothing to sneeze at.
My 9mm Corbon 125gr +p JHP defense ammo is serious stuff.

It probably goes against all the macho "one shot stopping power" crap you'll hear but if I had to choose just one to keep it would ABSOLUTELY be the 9mm.