I think both the sb600 or sb800 are excellent flashes. The main differences between the two are that the sb800 has a higher guide number, it comes with a diffusion dome, has a built in bounce card, and has a couple of manual operation modes that the sb600 doesn't have. If you plan to use multiple flashes, the sb800 can be used as a commander, but the sb600 can't.
Personally, I love my sb800. I've got 3 of them and am very happy with them.