Upload & Sell: On
Check this review: http://www.The-Digital-Picture.com/Reviews/Canon-EF-8-15mm-f-4-L-USM-Fisheye-Lens-Review.aspx
If you scroll down to the comparison photos, there's one of the 14mm 2.8 lens. Notice the lack of distortion from the 14mm compared to the fisheye images (both from the 8-15 and the 15mm fisheye). Even at 15mm on a FF (and a crop camera) the lines are still very much barrel distorted inherent to the nature of the fisheye lens.
If the fisheye curvy look is what your going going for, then by all means get the fisheye. But, particularly on the 5D2, the 8mm range on the 8-15 is a perfectly circular image. That's a pretty stark/limited use type shot. Lot of money to spend on a lens of that nature for the limited use, I personally would get out of it. YMMV.
I'd say, if you have the funds, go for the 14mm.