How wide angle do you need?  With that camera you also can use EF-S lenses so you have a couple more options.  If you want super wide angle, you could get the EF-S 10-22 mm lens.  It's pretty sharp and is small and light weight if you need to carry it around.  Perfectly fine for stock - I use mine all the time.  That one gives a focal length equivalent to 16-35 mm on a full-frame camera and it's about the same price as the 17-40.  If you're shooting large subjects in tight spaces it's very helpful.  The other lenses you mentioned are equivalent to around 25 or 27 mm so not nearly as wide.  There also is the EF-S 17-55 f2.8 lens.  It is very sharp but has a problem with CA in my experience.  I use that one a lot too but have to pay special attention to remove fringing in PS.

I think the choice will depend on how it will be used.  If for travel then small and light weight may be issues - those are very important to me.  If you think you'll be doing a lot of hand-held shots in low light then f2.8 might be important.  For me I almost never shoot wide open so it is not such a big issue.  Good luck with your decision!

I have the 17-40 F4, and it is a perfectly good lens. I don't use it a lot though - I tend to go 24-105 first, then then 70-200, then, occasionally the 70-200 with 1.4 extender, and finally the 17-40 in terms of usage. I do carry it everywhere and sometimes it is the only solution, but you can also pan and stitch that works well for wide panoramic shots.

Bottom line - don't spend more than you need!

steve

I have them both and my experience is very positive with both of them. The 16-35 is slightly sharper at the widest end, corner to corner and one stop faster if you need it.  In practice, that one stop faster is really the only advantage and will also set you back lots more dollars.

If you wanna have a good wide-angle don't buy Canon.
I have the Canon EF 16-35/2.8L II USM the 20mm the 24mm, I have used the 14mm and 17-40mm and the all are disappointing!The only decent Canon wide-angle I have is the 28mm/1.8 but it's not very wide and very soft at the edges if used wide open.
The Tokina 16-28 is supposed to be very good and its cheaper than the Canon EF 16-35/2.8L II USM. The Canon EF 16-35/2.8L II USM is certainly disappoints  for it's price I'm regretting that I have bought it.

As I said I would go for the Tokina 16-28 of course for Fullframe!
Consider the much smaller price but better performance than the Canon 16-35mm, wideangle is a general weakness in the Canon-system therefore many use manual wide-angle lenses (Zeiss for example). I'm pretty sure the Tokina is the best offer with AF for Canon Fullframe you can currently get.

http://www.photozone.de/canon_eos_ff/595-tokina162828eosff

Photozone-Tests are generally very good and reliable worth a read.

eclaire would you go for it even if not fullframe? I have a 50d

it depends if you are planning to go fullframe anytime in the future I certainly would buy fullframe lenses now. If you are certain that you will stay with the 1.6crop than you shouldn't buy a fullframe lens because they are more expensive, heavier and bigger. I don't have personal experience with crop-lenses but I would recommend the Photozone site to do a research for ww crop-lenses with Canon mount.

A 50D is a crop camera so something like a 10-22mm EF-S lens would be super wide

A 17-40mm is an EF lens and I have one for my full frame 5DMII so it's super wide on a full frame camera.

However, a 17-40 EF lens on your 50D would really be much wider than a standard 18-55mm or 18-200mm EF-S zoom.

Probably kind of confusing but do some research on EF-S vs EF lenses and full frame vs crop.

If I were you I'd go with something like the 10-22mm for your 50D.

Regarding a general statement like Canon zooms being soft, no. Some are sharper than others. My 24-70mm is awesome sharp. Every manufacturer has some sharp and not-so-sharp lenses.

If you wanna have a good wide-angle don't buy Canon.
I have the Canon EF 16-35/2.8L II USM the 20mm the 24mm, I have used the 14mm and 17-40mm and the all are disappointing!The only decent Canon wide-angle I have is the 28mm/1.8 but it's not very wide and very soft at the edges if used wide open.
The Tokina 16-28 is supposed to be very good and its cheaper than the Canon EF 16-35/2.8L II USM. The Canon EF 16-35/2.8L II USM is certainly disappoints  for it's price I'm regretting that I have bought it.

bad experience but no I wouldnt say they are that bad. When buying optics you have to try a few samples. It took me two samples to find a good 16-35 and a 17-40.

Its true though what they say. Nikon for wides and Canon for teles.

I would recommend the sigma 14 mm, 2,8. i bought it used on ebay and Im very happy with it.

It takes pictures like this:
http://www.naturephotos.dk/NaturePhotos_show_pic_ejergalleri.php?kgf=21298&art=0&menu=1&_Ejer=3&sort=&start=15
or this:
http://www.naturephotos.dk/NaturePhotos_show_pic_ejergalleri.php?kgf=18546&art=40&menu=1&_Ejer=3&sort=&start=38