Is it OK, then, to turn this into a Tokina 11-16 vs Canon 10-22 thread? It'll likely be a mix of indoors, landscape, outdoor buildings, large group shots and experimenting.
I have the canon 15-85mm as well. Can't yet afford a FullFrame and still waiting for excellent WA zooms there
(OK 14mm + 16-35 would be a great combo)
I went for the Sigma after reading a review or two. Most revealing was the combined reviews at http://www.the-digital-picture.com
The reason for my choice - nmy personal travel photography, capturing the inside of some buildings, especially churches, without the effort of stitching etc. Yes it's slower, but I'd rather rest it for one shot - bracketed or multiple shots if I am worried that I may shake, as oppose to worrying about lining up shots for stitching.
For what you list, and having the 15-85 already, I wouldn't necessarily think that the Sigma is the best.
(Plus no easy filter fixing) The Tokina has slightly more CA than the others. The Canon will probably hold it's value longer. Prices are similar enough not to get too hung up on the difference. Otherwise from your list
Indoors - unless you need extremes (Sigma) then the Tokina at f2.8
Landscape - equally good - Canon might edge it due to a mm wider and less CA
Outdoor Buildings - Canon edge as above
Large Groups Shots - all OK.
Experimenting - Sigma for the angle - (Tokina for the f2.
You pays your money you takes your choice...