Buy one of each!
I own one of each. I will say each one is more than capable of winning on any given day. Their are some differnces between them though. These are just my opinions of them.
Xray- (+)light, agile, built very well. (-)maybe not the most stable of the group in the air, but it does respond very well to inputs in air.
MBX-6, (+)light, built really well, decent amount of turning, tons of suspension travel. (-) On a tight track with lots of bite it can be a handful.
RC8B, (+) tons of steering, battery fwd & new towers help the handling vs. the old RC8, seems to handle ok even when the setup is off or coditions change. Favorite of the bunch for handling. (-) Fuel tank, just a tick behind the others for reliability.
2.0, (+) Seems to be the most stable of the group. (-) Probably not as much steering as some of the others.
The Xray and Mugen are built very well. Don't be decieved into the idea that they are maintenance free though. They are all very good cars.