Choose UWA for crop or FF?

I have been using a combination of a 5D classic and a 7D this summer and it's been great. Although I have been getting along with 24mm on the wide end with my 5D, I plan to buy an ultra wide angle lens in the somewhat near future. Should I get an UWA for my 7D or my 5Dc? I plan to keep a crop and FF sensor camera.

For full frame I would get a 17-40mm. For crop there seem to be a lot of good options right now. 10-22mm, 11-16mm, 8-16mm, etc. I've owned a 10-22mm in the past and enjoyed the heck out of it. I used it for landscapes, city shots, building interiors, and some interesting people shots.

17-40mm advantages: Better image quality on 5D (and eventual 5D2/5D3). It will work on the 7d. But I don't think that I would use it this way very often.

Crop UWA advantages: The 7D clearly performs better in all aspects except the final image over the 5D. Would it be better to have my UWA lens on my camera body that focuses/tracks better, has a built in flash (just in case), has live view, does video, etc?

I guess I asked myself the same question when deciding to sell my 15-85mm for a 24-105mm. In the end, I wanted to be able to get a little bit of shallow depth of field on my walkaround lens so the 15-85mm was out. But it does feel weird to have my most used lens on my slower performing camera with the 7D put away in the bag. But I do use the 24-105mm on the 7D as well. Maybe the best combo is the 7D/24-105mm with 5D/17-40mm. Any thoughts?

Your pop-up flash will not cover the field of view of an UWA lens, and UWA is one place where focus speed and tracking ability are of little use.

I think it comes down to two considerations:
- which combination do you feel will give the best IQ?

- Does your application of UWA need shallow DOF or will you be shooting stopped down?

Thanks. I knew I could get some help from my home town! I hadn't thought about the pop-up flash not covering ultra wide. But live view is a definite plus for landscape type shots.

I don't need shallow depth of field, but it may be in lower light situations that I use it. I think the 17-40mm is the way to go. Unless a rumored 16-50mm F4 IS comes along. I think that would be an easy choice because I would use that on the 7D as well.

I'm not expert but, I'm running a similar setup (7D and 5DmkII). The 5DmkII was my first full-frame Canon so I came from a collection of mostly EF-s lenses including the EF-s 10-22mm. If for no reason other than just to avoid the possibility I might accidently mount an EF-s lens on the 5D, I dumped all my EF-s lenses.

I got the 5D back in May and I've settled down to have the 17-40L as the lens that largely lives on my 5DmkII while the 24-70L is what stays on the 7D most of the time. The result is the 7D takes over with the 5D leaves off in terms of focal length with these lenses. It's been a great combo on photo walks with a buddy of mine in the city. Occasionally the 24-70L stays in the bag with the 70-200mm f/2.8L IS mounted on the 7D.