It is similar to saying, "My right arm is stronger than my left arm, and I have to lift a pocket knife and a set of keys - which hand should I use for each?"

Either servo is going to be more than enough to do any of the jobs you're tasking them to do.

That said, if it is a tail dragger, I would use the 635's on the ailerons. If it has a nose wheel, I'd use them on rudder and elevator.