The primary time you'll notice the difference between 5DII and 7D image quality is if you do very large prints and/or crop heavily.
There is more control over depth of field with a full frame camera, but that's much less of a consideration when shooting with very wide lenses.
I use 12-24 Tokina on my crop cameras and 20mm prime is my widest on full frame. Both are pretty darned wide. For example, the 2mm difference between the Toki 12-24 and Canon 10-22 is equal to approx. 2 or 3 steps backward. A less radically wide lens, or one that doesn't have a really large (unnecessarily) aperture, is often more easily or better corrected.
Optics are always about compromise. You can even see it in Tokina's lens line-up. The 12-24 is an f4 lens. The f2.8 of their 11-16mm attracts a lot of people, but note how narrow the zoom range becomes. I also feel few will actually need f2.8 on such wide angle lenses. Most of the time we're stopping this type of lens down, not using it wide open. Certainly we aren't expecting to blur down backgrounds very much with it. Low light might demand a larger aperture, but today's cameras are higher ISO capable, offsetting to some degree the need for larger apertures (and I.S. for that matter, especially on shorter focal lengths).
Don't rule out third party lenses just as a matter of course. In some cases they are better built than Canon's offerings and offer equal image quality, and sometimes for a good deal less money. The Toki 12-24 I use is an example... built like "L" series I've owned and used, equal in IQ to, but costing about $500 as compared to the EF-S 10-22 for $750.
If you are looking for top image quality and still considering a cropper, be aware that the T2i, 60D and 7D all produce identical images with the same sensor and image processor (7D has dual processors to allow it to shoot at higher speeds). A lot of people buy the 7D simply because it's gotten such rave reviews, without evaluating if they actually need or will even use it's features. It's essentially designed to excel at sports/action. This doesn't mean it can't be used for other things and do them very well... It just means you are buying and paying for features you might not need.
The considerably less expensive 60D and T2i both use the same, new, 63 zone metering system of the 7D, too. They have simpler AF systems, which are straightforward and good performing. A lot of people new to the 7D find it confusing and difficult to work with.
5DII is a nice camera and very capable. It's AF is quite similar to the 60D or T2i, just adds a few hidden "expansion" points that the crop cameras don't have. If you are for some reason wanting ultra wide angle, the Sigma 12-24 is a full frame lens that sells for around $750 (The Tokina 12-24 is technically a "crop" lens, altho I can tell you it works on FF up to 18 or 19mm). This Siggy is the widest non-fisheye available from anyone. I think it compromises a little on image quality, to go as wide as it does. But, that's typical.... Optics always involve some compromise.
If you have your heart set on a 5DII and Canon 14mm.... You might want to rent them first. I agree that the full frame camera with the 17-40 or 16-35 would be more versatile and generally useful.
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