My biggest problem with Ultimarc's interface is that you need a separate USB input to use it. This doesn't make sense to me since you need an ipac or similar interface for the up, down, left and right microswitches on the joystick and your pushbuttons.
The GGG interface is integrated with a gamepad-type interface and is a complete solution. But if you already have a keyboard encoder, you'll need to remove it.
To me, for swappable control panels, the type of encoder (such as rdagger's) which connects to the rotary sticks and needs to be wired to an ipac or wiz type encoder is the perfect solution. If I didn't already have one, I'd be a buyer.