I've owned both, the 6400 is a better looking set, the bezel is much thinner that its sibling. The 6400 is put together differently than the 6000, so less chance of it having the dreaded 'FlashLighting' effect(my 6400 has zero FL, it's a H301 panel).
the 3d on the 6400 is NOT FULL 1080P, if 3d isn't your thing no big deal then, as reg 2d 1080p is gr8! and even SD on either set looks gr8!
there is a comparison tool @ samsung.com or u can go to the 1st post on the 6000 owners thread and it'll lists all the differences.
if u save $$ by purchasing the 6000, then get it. additionally, the bezel is about an inch less on the total width for the 6400, so if you're trying to put it in a tight space, that might be to your advantage too.