However the 5d mark II is down in price. Should I go with the 5d and save the money for an additional lens? Do you think it will go down even more when the 6d is released in December?
The big advantage of the 5d2 is that it runs Magic Lantern which adds a decisive edge esp. for video, it won't be available for the 6d for some time (if ever). And since both don't have swivel screens and are outgunned by newer and less expensive video-only solutions the cheaper (used?) 5d2 should be a good choice. The only known problem is aliasing which has been fixed on the 5d3 (and probably on the 6d, too, we'll see).