I have a GT-10B which i am very happy with, i can't speak for the ME20B, all i know is the ME20B has just a basic set of effects & things.
The GT-10B has a whole lot of stuff inside it, the preamp sims are superb, great compressors (even a multiband comp), massive E.Q options (you have up to 5 sets E.Q to use at once).
The USB feature allows recording and music playback via the USB into the GT-10B, and provides a midi connection for patch editing and librarian software for sharing patches.
The patch change can be set for smooth transition, so long as the next patch has a simular setup of delay and effects chain order.
The GT-10 also has a parallel effects chain, and all the effects are moveable anywhere, even the external loop, so you could use your external bass amp's preamp as one of the moveable effects blocks.
