If you are willing to pay more for a lighter bike, go for the 3700. Nobody can tell you whether the difference is worth your money, only you can decide that. I would opt for the better one (in fact, I chose the 6700)
I think I've narrowed it down to at least two mountain bikes, the Trek 820 or the Trek 3700-the $380 one. Theyre basically the same bike, except the 820 is steel and the 3700 is aluminum. Which should I get.
It realy comes down to what type of riding you will be doing. If you ride on mountin bike trails i would definatly choose the 3700, beacuse it is aluminum. The 820 is cheap but not that as nice of a bike (cheaper components and its heavy). I personaly like Gary Fisher bikes, but there are a bit more expensive, and i do ride quite a bit. I would definatly choose the 3700, its a nice bike, and remeber you get what you pay for.
Hope this helps