Canon Fisheye Question

Hi All

I am shopping for a new lens and I have a couple of questions. I shoot the 5DMrkII (full frame camera) in an Ikelite housing with the 8" super dome.

Money not being an issue in my decision process....

I am looking at the Canon 8-15mm vs the Sigma 15mm

1: Which lens would people recommend? Any pros/cons to one vs. the other?

2: If I shot the exact same photo with the Sigma 15mm and the Canon 8-15 at the 15mm end and placed them side by side would they look the same?

Any advice is greatly appreciated.

Thanks!!