As illustrated in the post I made above, the original 5D has a better T-stop loss than the 5Deuce.
F/1.4: 5D -0.25EV; 5D II -0.45EV
F/1.2: 5D -0.4EV; 5D II -0.6EV
Steam is running a halloween sale: Halloween 2010 Sale
Get Plants vs. Zombies GOTY edition for $5 (cross platform) and watch your productivity grind to a halt.
Even with the high ISO capability of the 5Deuce, I'd still opt for a fast prime. Personally, I'd go with the 35mm, for it's greater FoV @ F/1.4 vs. the 50 or 85mm.