how much "fisheye" effect to expect with the canon 16-35 mm zoom vs 14 mm prime lens

Both are very well corrected.

None. You would need a fish-eye lens for that. You will however get wide-angle effect. But you cannot get around that as it is a geometric problem with optics and a flat film plane that connot be eliminated in the optical design.

Visit this thread. At the bottom of the page, you'll find three photos taken on a 1Ds with the 16-35 f/2.8, 14 f/2.8 and 15 f/2.8 fisheye.

No, you won't, the images have been taken off the web.

But here's an image taken of yours truly with the 15mm f/2.8 on the 5D that should illustrate how that one works quite well. The right side is cropped, but it doesn't matter here.

I'll post an edit if I find an image I can use that was taken at 16mm with the 16-35mm f/2.8.

My bad! I should have stayed outside today

The 16-35L and the 14L are designed such that straight lines will be reproduced as straight lines. I don't have the 14 but I do have the zoom and while it isn't perfect, the distortion that does exist at the 16mm end is pretty subtle. You shouldn't find it at all objectinable. And it's easily corrected in software.