The difference between the 800 and 850 is about the difference between the 700 and 750. The 850 has got a better frame with much better components. 850 should be double-butted and made in the USA, while the 800 will be made in Taiwan from cheaper tubing. I'd swap the fork on the 850 for a rigid one since early rock shox elastomers were very flexy. The 850 has a threaded 1'' steerer.