I've shot both, for underwater and for commercial sales. Underwater, I compared the 5dmk1 and the 40D, same housing, same lenses. I posted them here on WP and took out the exif. Not sure if I added 100% crops. Honestly, it was a draw. Currently, for my commercial clients, I've shot the 5dmk2 and the 7D. Tripod mounted, cable release, studio conditions. Once again, blown up to my client's spec of 16x20, 300 dpi, you might...big MIGHT be able to see the difference of the 5Dmk2 in well lit conditions. In lower light, sure, I'd give a stop, to a stop and a half to the 5dmk2. For underwater though, it's hard to give up the flexibility of the Tokina 10-57 as well as the added reach the crop gives a 100macro for flighty critters, add a 1.4TC and you're at 225mm equivalent FOV to a FF camera.
Only reason I'd consider housing the 5dmk2 right now is it's the spare body, the 7d is much friendlier for UW use.
Stu