Wow, that's a tough call. Both are great cars with manufacturers that have put a lot of time and development into the car. ANY buggy is going to have an issue here or there... I personally just keep "common failure" parts on hand for my chosen ride. Break something, well, put the one on it that's in my parts box, and get another one to replace the missing spare. The ONLY "high failure rate" part I've heard of on the 808 is the bearing on the pinion gears... at $1 each you can buy a LOT of bearings for the cost of a different kit.
If you are happy with the 808, you'd probably be better off spending your money ordering some spare parts and sticking with it. The MBX6 looks really good, although only one guy locally has it that I know of so far... 808's are like belly buttons around here, so SOMEONE would almost always have a spare to swap/sell/whatever.
If I were shopping for a buggy... I'd probably call the guys in the white coats to come take me away, because I'm not a good enough driver for buggy!
Seriously, 2 great cars, it would be hard to choose, pick what you like and if you have no local parts available, just stock up on your own.