I can't decide what to get. Can anyone help me decide?

I want a diagonal fish eye for my 5Dmkii. I'm not too interested in getting the circle look where the black makes the subject trapped in a circle, I just would like the distortion at the edge of the frame.

I was looking at the Canon 8mm-15mm f4L fisheye for $1290 but that only fills the frame at 15mm.

The Sigma 15mm F2.8 EX DG Diagonal Fisheye is less than half it's price at $603.

I've never used Sigma before and all my lenses are Canon, so I'm reluctant to buy anything but Canon until I know what I'm getting myself into. I'm leaning towards the Sigma because I won't be using the fisheye as often as my 24-105mm or my 100mm f2.8 Macro, but I'm not sure whether it's worth forking out the more for the Canon L. Is it much sharper?

Has anyone used this lens before or had any experiences they'd like to share about Sigma?

Thanks

Matt