5D2 if you regularly shoot and make large prints at ISO 1600 and above. OR if you have certain Canon lenses which really work best on FF (T/S; the fast wide primes like the 35 f/1.4).
7D for everything else.
No matter how many times people insist...just insist...there's a HUGE difference when they pixel peep at 300%, they cannot pick out 16x24 or 20x30 prints at ISO 100-800.
And $600 would make a big dent for lenses, either buying one you need out right, or getting you that much closer to one that's more expensive.
But...if you must buy a full frame, I can't imagine why anyone would buy a 5DII when the 6D can be had for essentially the same price and you aren't buying four year old technology. In fact, right now, a new 6D with the 24-105 kit lens is actually cheaper than a new 5DII with the same lens. (B&H and Adorama).