Hunt for an UWA lens

So I'm in the market for a new lens, particularly something on the wide end of the spectrum for my 1dmk2, I shoot mostly winter action sports, but would enjoy getting into landscapes in the future. My budget is around 600 USD, and so far I have narrowed it down to these 3 lenses

canon/sigma 15mm 2.8 fisheye
17-40 4L

From what I have been told, the 17-40 is not all that sharp for being an L lens, but the zoom on it would be very useful in some situations, plus it would bridge the gap between this new lens and my next widest lens(50mm). How ever, the fisheyes are often said to be a must have for any ski/snowboard photographer, but the effect the lens has on an image might get boring after a while, which is what I'm afraid of. What would some of you recommend?

The 17-40 is plenty sharp in the middle. It might suffer a tiny bit in the corners, but if corners were an issue the fisheye wouldnt really be a consideration anyway. To me the fisheye is a one trick pony, a dramatic trick yes, but imagine a gallery of nothing but fisheye shots. You can do some wow things with it, but after you show someone a dozen they begin to get the impression that the lens is the wow and not the photographer.

You'll generally be shooting stopped down. The 17-40 never has perfect corners, on a FF, but if you keep that in mind, you can crop those out. Knowing that, frame appropriately. No idea how the corners behave on a 1.3x

I actually prefer to use crop UWA lenses on my 1.3 camera. Like the Tokina 11-16. It works from about 12mm to 16mm, which actually does give a true UWA view. I did not find the 17-40 gives a wide enough FOV on a 1.3 crop. The crop lenses are smaller, lighter, and generally cheaper.

I have a Sigma 12-24 UWA, the widest FF available, that I am selling. It's a great lens but I don't landscapes enough to justify keeping it. PM me if you are interested.

If the Tokina 11-16 will work as wide as 12mm, you might want to check out the Tokina 12-24/4. It's well within your budget new (about $500) and widely available for about $400 used. It's technically a 1.6X/1.5X crop sensor lens (in Canon and Nikon mounts), but I've used it as wide as 18 or 19mm on FF. I've never tried it on a 1.3X crop camera like yours.