You get what you pay for. The 800 has lock out but i would want BB7 disk breaks instead of hydralic. I would go with the 600. It has some better stuff than the 400 but the price jump to the 800 just seems like too much. You get a better rear deraillier for the 800 but you can buy a 2003 (i think) model XT on Jenson for $39. I couldnt find right now the weight for each bike, see if the 800 is lighter. It might be heavier because i could have beefyer components. I would go with 600.
SScomp, I just got back into cycling a couple of months ago, learning to ride MTB, because street traffic is too dangerous for me at my age of 53, lol! The Cannondale Demo Rep was at a local park with about 30 bikes, half mtb and half for rode, so I spend a couple of hours checking out the equipment. I rode both the Prophet and the new Rush. I had never heard of a 'lefty' and I was quite impressed with the design, so much so, that I decided to order a 'lefty' bike. I preferred the Rush over the Prophet because I am just riding trails and learning as a beginner to ride rough terrain. The Rush was lighter and more nimble, seemed to accelerate quicker, and didn't stand as tall as the Prophet. Can't wait to get my bike completed, I ordered a frame to build up but the Fox Terralogic fork that I ordered has not arrived yet.