With servos, there are 3 things to consider.

Torque - you need enough force for the servo to move the control surface through the desired throw, and keep the surface there. Keeping in mind that standard servos are prone to "blow back" because they only develop their rated torque when moving. They don't develop the same torque when trying to hold a position. (if you have a servo you don't mind messing with, try bolting on a long horn, then try fighting it with your fingers. You'll find it's not hard to move the servo a small amount from any position, but the farther you move it, the harder the servo fights back). Max torque occurs when the servo is farthest from where it wants to be. Digital servos fix this problem. They produce nearly max (or max) torque even for small deflections. (which is why they suck battery power like crazy, they are always working "full power"

Speed - You can get servos that respond faster to your contorl imputs. You might not think this is a big deal, but depending on the plane, you can actually feel when the controls move faster. This is espeically helpful in some acrobatics, including 3D

Precision/resolution - This is really several things that I'm lumping together to keep an already long post from getting truely epic. Basically, some servos center more accurately than others, some go to the same end point more consistantly than others, and some have smaller "dead bands" than others. Deadband is the range of stick movment that results in no servo movement in response. In theory you want 0, but in reality, you need a little deadband to keep the servo from constantly hunting or jittering. (at least in standard servos. I don't know how deadband affect digital servos, I think it's the same, but I don't know for sure).

Anyway, you can pay more for servos and get better torque, better speed, and/or better resolution. Which might or might not make some acrobatics easier (or possible).

Picking servos is something of an art and heavily based on personal preferance though.