I've owned the 17-40 for 1.5 years, and I think it does the job. I'm not quite sure about mojo in UWA - which, if I had to say, I would assume it to be overall technical excellence (i.e. sharpness across the frame, micro-contrast, colour, distortion, etc.). IMO, unlike fast telephoto, where you can like a lens independently from the technical specs due to its "magic look", UWA is really dependent on technical specs.
Anyway I was considering these two lens back then, but after some testing, the only determinant factors for me were aperture and price. Will you shoot at 2.8 a lot? Perhaps lot of low light work, environmental portrait? How much would you like to spare for UWA?
If you seek for the best quality, then primes (TS lenses especially) are really good.
Now I mainly use it at f8-f14 for landscape and travel with tripod. And so far, I've been satisfied with the lens.
Some samples:07APR12 - Ruins to the Dom 3
, on Flickr[EXPLORE 10.11.2012] Venice 2
, on Flickr