wide angle choice
currently own a 30D and 1D MarkIII and im in need of something wider
than my 24-70L.
i was thinking of the tokina 11-16 2.8, but then im limited to
sticking it on the 30D... or do i got with a slower sigma 12-24
F4.5-5.6 and stick it on the markIII whenever i want?
I would probably go with the sigma 12-24. I use it on my 5D and my 40D and am very surprised by this lense. While it isn't as wide as the Canon 10-22 or the tokina you are looking at, it will fit on both bodies. There is not a whole lot of lense distortion and you will get some very interesting shots with perspective distortion! Ultra wide is great. the speed really isn't that much of an issue for me.
Now that I have said all that, what is it you plan on using this lense for? I only ask because if you have a specific purpose in mind, there may be considerations to think of other than it fitting on both cameras.