The cost difference in the 3003 vs the 3004... may as well get the 3004. (its only a couple dollars a servo...)

The ball bearing gives less play at the output shaft. A very good thing for Flutter prevention. If that were the only difference it is worth the extra cost. But the ball bearing also has just a bit less friction... so the servo travels just a bit faster and gives JUST A LITTLE crisper response. (you gotta be really good to see the difference in flight...)

Match the servos used for ailerons if using 2 servos. and as you get replacement servos... go with the 3003 over the 3004.