Agreed - the outside of the WDW version is certainly way superior. However, initial reports are saying the actual ride portion has better show elements - so we'll need to ride it to find out I guess!
Word has it that they removed all of the signs and neon from the inside of MGM's RNRC and replaced them with a bunch of fog and synchronized lights. It certainly sounds intriguging.
Maybe i can help you with your decision. I've done both and i must say that i like the one in WDS better than the one in WDW.
Agree the outside looks not so good than the one in WDW but once in the cars it's a whole new experience. It's much lighter inside, you can see the tracks better. Beautiful lighteffects and a very smooth ride.