If you are going to use the TTL function of a Canon camera for a 5D MII (which I have) you need to purchase a converter ($$$) whereas a 7D you can use the pop up flash on the camera inside the housing and use fiber optics. There is less chance of flooding the additional electronics of the converter and cable and the optics seem pretty reliable from everyone that I have seen using them on various cameras out there.
If you use a 7D with its pop up flash your battery life will not be as long - just set the output to minimum however and your battery life will be fine.
I have used by 5D MII with an Aquatica Housing and have been able to shoot for several days without removing the camera from the housing because I don't have a flash going and I shoot mostly manual now that I have more experience. The TTL comes in handy when you need to get off a quick shot.
The housings for a 7D are smaller, lighter and with no TTL converter there is more room in your bag with less chance of exceeding weight limits on airlines. The 5D is a bit better for low light conditions over the 7D but I still find that I need focusing lights to help with the focus with the 5D MII. This includes those days that are overcast vice bright sunny days.
This is from my personal experience with my 5D MII. I have not taken my 7D underwater but have discussed this and read up on the 7D before my purchase of the 5D MII for underwater use.
Both the 7D and the 5D MII use the same lenses so the quality is there for both.
Hope that helps.