Compact or full size? Ammunition...both will work fine, 9mm is cheaper. Like has been said, it's not what you're shooting, it's where.......I owned the as model in .40 cal. full size. The trigger was great, the nicest thing about the firearm. The rest, I did not like..it did not fit me properly, even with the changeable backstraps...the recoil was too much, which is a usual for a polymer firearm....some internal parts were "plastic", which I didn't like. What I would suggest, if you haven't already, is to go rent and fire each one, if possible......different trigger systems, different animals.....and try some other firearms as well...not to talk you out of a P99, but after you check out some other firearms, you may find something that fits your situation a lot better. With some practice, either one would work well, for offensive or defensive applications.