I haven't owned or driven an MBX6, so I can't comment from personal experience. What I've seen from the racers in this region who have them all say they like the way the car drives and I think they look fast on the track. I've seen some of them break the standard front upper hinge pin plate and rear lower hinge pin block. Drivetrain and suspension quality look good like Mugens always have.
I know that the RC8 picked up the reputation for bad fuel tanks, which was a bummer. We're on the third generation of the tank now and I'm seeing a lot fewer people saying they're having problems. The RC8B has all kinds of small improvements in the car that customers were asking for, as well as the new shock towers, big bores, steering mod and battery relocation. It's a very nice piece. The diffs have been outstanding for me ever since I got my first RC8 a year and a half ago. The big bore shocks have quality that will rival any manufacturer. The new shock towers and hinge pin plates allow more adjustment choices.
Lots of the AE team guys are prowling the net and are available to help with questions. Every time I've changed a setting on my car, it's responded noticeably, so if you want the car to do something different, you'll be able to set it up to do that. I've run setups on my car that have gone from very forgiving and easy to drive, to psycho quick and responsive. You'd be able to get the car to handle however you like.
I think you'll like either car, but you really ought to run an RC8B.
