Wide angle Vs Fisheye

Is there any advantage in buying N mm fisheye lens over ultra wide angle lens with the same N mm other that price?

I believe a 15mm fisheye will give a much different perspective than a 15mm wide angle lense.
A true fisheye should give about a 180 degree field of view, You wouldn't get that from a wide angle.

This was shot with a 15mm fisheye. Notice at the end the garage door looking like a tunnel from the fisheye distortion.

I have exactly that same lens. Everything not on the center horizontal line looks leaning. I was hopping that this does not happen with an ultra wide angle lens, while displaying the same area.



Of course in reality, all columns of this temple are straight and vertical, but not when pictured with fisheye.

In an ultra-wide lense. It would have some distortion in closeup lines. but not to the extent of distant objects.

I've actually been wanting a wider angle prime and was thinking of buying one of these
http://www.bhphotovideo.com/bnh/controller/home?O=WishList.jsp&A=details&Q=&sku=12082&is=USA

Or the Sigma, It's 1.8 so would be better in low light. and has some pretty good reviews.
I'm afraid of anything wider with the 5D which may have some vignetting problems.

http://www.bhphotovideo.com/c/product/217794-USA/Sigma_411101_Super_W_A_20mm_f_1_8.html

Of course you could go with a ultra-wide tilt shift lens. It would give straight lines but they are very expensive.

Well, I was thinking about the http://www.adorama.com/catalog.tpl?op=detailsdisc&sku=SG1428SG 14mm 2.8

I saw that one too but read that on the 5d it had vignetting problems. Maybe not.
That's one reason I was staying with a 20

Thanks, good point.

This one was shot with SIGMA AF 12-24mm f/4.5-5.6 HSM EX DG lens at focal length 12mm. The length of horizon line is about 1km.

Zygis

Is it cosidered "prime"?