I've put Dave Brown retracts in a couple of planes. They're mechanical. You have to have a very straight route from the servo to the retract (you can get by with a bend if you have to) but when they're locked up and locked down they're very solid. Air retracts (I've put one set in) are bigger because of the cylinder, more expensive (as you said) but the air supply can be anywhere in the plane, including in either the wing or the fuse. Quick disconnects work ok. They lock by putting continual pressure against the retract toggle so they probably hold "harder".

A lot has to do more with the weight of the plane, the weight of the gears and tires, and how much you're willing to cobble up the wing to put them in.