you want the rudder as far back as possible if you can get it behind your prop that's the best. On my walmart sub i'm running a motor connected to a pushrod connected to rudder. To keedp the motor from spinning in circles i screwed to little screws into some holes on the case so the control arm hits them before reversing throw. I"m still working on a centering setup.

As for the differntial thrust setup that would work great too that's what the radioshack sub uses. I dont like the way it looks however.