The key word here is future 3D airplanes......the bigger the better.......who knows what you'll fly in the future.....HiTech makes a nice servo HS 5645 MG.....
You'll generally going to want a beefer servo on the rudder...bear that in mind when you buy servos.....
Tis better to have a bigger servo than you need than a smaller one than you need....
Good Luck