As has been stated already, the pre 02 which is 600 c.c. is a totally different machine than the 02 and newer with the latter being more refined. Either work well and I would think you would be happy with either but the 02 and newer is truely a step above. As far as value goes. all I can do is give you an idea of prices around here. The 600 Grizzly in average to good condition would run $3,000 to $3,500. An 02 660 in average to good condition $4,500 to $5,000 and 03s and 04s going up in value accordingly.
As I've said here before, I'm a Kawasaki fan, but on my last purchase the Grizzly 660 was my next choice. Either way, it's amazing what you can do with these new big bores, both work and play.