the tamron 17-50 wouldn't be mutch wider then the kit lens the t2i comes with. Did you buy only the t2i body?
This lens the sigma 10-20mm is a very wide angle lens and whats good about it is you don't get the fisheye distortion
If you do want the fisheye distortion then i would go with the Rokinon 8mm fisheye
I have the rokinon 8mm for my t2i and love it. Its extremely wide and can see about 160 degrees around
check out this edit I made with it, all my other videos filmed with the t2i that arnt a fisheye were just shot with the kit lens 18-55mm