Fisheye Lens Help Please

Hi All
Can I have a bit of help and guidance please.
I have a sigma f2.8 EX DG 15mm fisheye which I use on a 5D Mk2 and I have great fun with it and find myself using it quite a lot
The opportunity has arisen to buy a Canon 8-15mm f4 L fisheye, I am tempted but apart from the circular fisheye effect and being able to use it on my 7D are their any other benefits to the Canon which would justify the extra cash
BTW I have never tried astrophotograpy so the loss of one stop between the two is not of significance.
Thanks in advance

The Sigma focuses closer, but not by much. Σ15=5.9", Canon 8-15=6.2". That, f/2.8 and about $800 difference and I'm happy all day long with my Sigma 15mm f/2.8 FE.

May I suggest a low cost alternative to the Canon L fisheye - the Rokinon/Samyang 8mm/3.5? Sold under various nameplates, it should cost around US$240. (I don't know what its price would be in the UK, where "troutfisher", the OP, apparently lives.) It is designed for a crop camera, but can be used on an FF camera. The newest versions have a removable lens hood that permits use on FF.

The lens is completely manual: focus and diaphragm. Must users suggest f/5.6 or f/8, as f/3.5 is not as sharp. At these apertures, almost everything is in focus. I just use the distance scale on the lens. My 60D and 70D both tend to overexpose with this lens. Just dial in -1.0 exposure compensation. For exposure I use the "Av" setting for aperture priority.

Results probably won't be quite as good as either the Sigma or Canon L fisheyes, but might be good enough. There is a lens sample thread for the lens in this form with many good examples. But I couldn't resist throwing in a few here. As the OP lives in the UK, I chose examples from a trip I make to Bath, UK, recently. The first is in the Bath Abbey; the second is in the Roman Baths.

RodneyCyr wrote in post #17624838
May I suggest a low cost alternative to the Canon L fisheye - the Rokinon/Samyang 8mm/3.5?

But the OP is also shooting a 5d2.

If a manual focus lens is a viable option, allow me to suggest the Rokinon 12mm f/2.8 Fisheye. It is sharp across the frame wide open. I'd have one if I didn't already own the Sigma 15mm FE, and just spent all my money on a 135L.

Thanks very much everyone for their input, on reflection I decided to stick with the 15mm Sigma as I could not justify the extra cash for the Canon