As long as the leather stays supple, it's a good idea for the mitt to retain some of its stiffness. Even pitchers need more than a piece of limp leather hanging on their wrist! Break it in properly and keep a couple of balls in the pocket whenever it's not being used. If it gets wet, use a little glove cleaner on it, massaging it real good to maintain that suppleness.
I've got the a3000 for my son and I, sizes 11.5 and 12 respectively. I'm 6'1" with 9" span (thumb to little finger) and 7.5" from wrist to tip of middle finger. The 12 fits fine for pitching and OF, but if I was still playing middle (SS or 2nd) I'd go with the 11.75" or even the 11.5" for quicker transfer.