RE: Servos For A 3D Gas Plane
hitcttd has nailed most of the salient points but I'd add:
Run a 5-cell (6V) flight pack and you'll find that your servos are faster and stronger, you'll *really* notice the differnce. That extra cell makes more difference than going from analog to digital servos and it's a whole lot cheaper too :-)
Don't use Karbonite on a gaser -- overhyping this material was one of Hitec's biggest marketing stuff-ups and they're still unwilling to admit that they got it very wrong when they claimed it was suitable for large models -- it aint!
On larger planes the speed of the servos isn't quite as important as on small nitro models. Big models respond more slowly anyway (due to their greater mass) so, if you're on a budget you might want to consider the HS645 servos which are very strong and acceptably fast on 6V.
If you've got some money to burn, just use HS5955 servos on everything. These servos *rock*. They are digital and have titanium gears. They don't reach the claimed 300+oz/in of torque but they're still way stronger than you'll ever need on anything short of a 40% model.
A mid-priced option would be to use HS5625 servos on elevator and aileron with an HS5645 on rudder.
But, to give you an idea, my 30% Extra 330L has:
4x HS645 servos (analog metal-geared) on aileron (2 per wing)
2x HS5955 servos (digital titanium geared) on elevators (1 per half)
1x HS5955 servo (digital titanium geared) on rudder
1x HS425 (analog nylon-geared) on choke
1x WayPoint 1620 (analog nylon-geared) on throttle
They're all way-fast-enough on 6V.