Well, a diagonal fisheye will give you the fisheye effect within a full frame, rather than a circular fisheye which will have a circle image within the frame, surrounded by black. Using a diagonal fisheye on a camera with a crop factor will cut off the edges but the fisheye distortion will still be there.

I personally went the cheapie route to get a fisheye because well, I'm cheap, and I wanted a circular fisheye instead of a diagonal. I bought a cheap Canon 22-55 mm zoom and a little fisheye converter thing that screws onto the filter threads of another lens. At the 22 mm end of the zoom it produces a full circular fisheye effect. This is with a 35 mm film camera. Total cost to me was like $150. It's not exactly the sharpest lens I've ever seen but for $150 it's plenty fine.