I'd only buy the Tokina 11-16 if you know that you'll need f/2.8; the Canon is both wider and longer and thus a more useful lens for landscape work. Personally, I own the Sigma 10-20 f/4-5.6, it has very solid (but not L-class) construction and is almost the equal of the Canon resolution wise at f/8 and above. Whilst it has better resolution, I couldn't recommend spending the extra on the Canon; if you're buying it the UK the 10-22 costs more than the 17-40 f/4L! That's just a rip-off in my book...
the EF-s 10-22 average price is just under £599, used price you can buy them for around £400/£450. The 17-40 L is around £620, used price around £570
so they're pretty similar, If I had a full frame body I would just go for the 17-40L (again, the f/2.8 of the 16-35 just isn't needed for the work that I do) but as I'm using a 7D the EF-S 10-22 is the perfect partner, I am still considering the Sigma 10-20 though, I think I'm going to rent both the Canon and Sigma for a few days and shoot as much as possible and work out the pros and cons of each