Aileron

In Wing

The Servo is mounted in a Servo bay and its aileron wire is run through the wing to a horn on the bottom of the wing.

Pros:

Most “clean”
More aerodynamic (I assume I don’t really know)
Servo is not exposed

Cons:

Hard to do maintenance
Can’t be undone (use of epoxy)
Requires additional mounting hardware
Difficult to manufacture

Top Wing

The servo can be inset in wing

Pros:

Easy maintenance
No additional mounting hardware required
Easy to manufacture (I think, I don’t really know how we make wings)

Cons:

Servo is more exposed
Less “clean”

Bottom Wing

Very similar to top wing mounting. It is less exposed to the elements but could be at risk of hitting the ground if things went wrong.

^This method can be used to mount on the top of the wing as well. It protects the servo a little more but requires a bay to place the servo.

Bottom Wing vs. Top Wing

As for mounting on the top vs. the bottom, I found all sorts of conflicting statements online. The only one with data-based evidence and not anecdotal was the one linked below. I can’t comment on the accuracy/validity of the experiment because I know nothing about aerodynamics.

Given the data collected mounting on the bottom of the wing provides the least amount of drag. with the exception of case 5 (short, open top linkage).