PS- I think the best thing about having a Canon is being able to use the Canon lenses :-) The 24-70 is a great all purpose lens (pricey b/c it is an L series lens, but goes to a lower aperture than some of the other zooms like the 24-105...though that one was recommended to me some too)
I would love to stock up on several more of the Canon lenses.....one day!
I've got both a Sigma 10-20 f3.5 and a Canon EF 16-35 f2.8L for my 5d mkii. I love them both quite frankly, but I think for the more wacky shot the Sigma obviously wins. If you really look closely the Canon has better quality, but then again about twice the cost......
I'd recommend the 16-35 f/2.8 (version 2) or the 17-40 f/4, both L lenses. But if you want something wider, then perhaps third party zoom as you have indicated. I used Sigma 12 - 24 in the past. OK. Fun for superwide, but not as impressive as the Canon's own (used 16-35 f/2.8 (v1).
http://www.the-digital-picture.com/reviews/canon-ef-s-10-22mm-f-3.5-4.5-usm-lens-review.aspx
http://www.flickr.com/groups/efs1022/
I would love to stock up on several more of the Canon lenses.....one day!
I'd recommend the 16-35 f/2.8 (version 2) or the 17-40 f/4, both L lenses. But if you want something wider, then perhaps third party zoom as you have indicated. I used Sigma 12 - 24 in the past. OK. Fun for superwide, but not as impressive as the Canon's own (used 16-35 f/2.8 (v1).
BTW, the new version of Sigma's 12-24 looks interesting..and they get rid of the "gold ring"(which I didn't like)!
http://www.dpreview.com/news/1102/11020804sigma12-24.asp