Basically whatever you prefer. If you want to be a serious competitor you may want to go with a 4 3/4. It'll clear leather a little faster but IMHO not fast enough to make much difference. If you're going to shoot for the fun of it just use what you like most. Go to a match and see what they're using. Chances are you'll have several offers to try out different guns after the match is over. Find out what you like and don't like before you spend a dime.
