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.