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Depends on how wide you want it. I only saw 2 at B&H the was the Cannon15mm f/2.8 for $580. Then there are 2 Sigmas the 15mmf/2.8 id $500 and, the 8mm F/3.5 for $680. So it is a matter of what you plan o shooting with it. Personally if I had a Cannon I would get the, Cannon lense even though it is $100 more.
Depends on how fishy you want it. On a XT a 15mm fisheye is usually not fishy enough. You might have to look into a 8mm fisheye. I recommend the Sigma 8mm....