Yeah, normally, I'd say upgrade your glass first, then worry about your camera body. However, a 50mm on a 5DmkII is a beautiful thing. You'll never go back to a cropped sensor. Honestly, though, this is a tough decision.
One more thought: Going from a 50mm f/1.4 to a 50mm f/1.2 is a marginal upgrade, at best, in terms of IQ.