I am a wide angle fan, but not a fish eye fan. But if you have them, why don't compare them?
The Competitors of today are the White Russian 8mm Peleng, mounted on a Nikon F4 (that’s an expensive combo), The Diana F+ with the 20mm Fisheye (that’s a cheaper combo) and the babyfish (that’s so cheap you just have to buy it to put on your shelf (I did)).
I always had my doubts when I saw Diana fisheyes about he the stunning 170 degree angle. To me, a 180° Peleng users it just didn’t look that wide. Yes, the curved fisheye effect was there. Now I know for sure, or 170° is a lot smaller then 180°, or the Diana doesn’t cover a 170 degree angle. I think the latter. When you do some calculations a 20mm lens has an 108° angle of view on medium format. So my eye did not decieve me. But as you will see in the examples below, this gives a totally different effect. As your photostream is not complete without a cow or a kitten staring at your fisheye lens, The Diana and the Babyfisheye do the job wonderfully. Because you can take close ups and have the cow (horse in my case) filling your frame. The Peleng is so wide that even when you put the camera against the animals lips, you can still see all the landscape surrounding it. Take a look.
Here I am holding the camera to the horses nose, and still the other one is in the picture.
This one is taken from the same spot. As you can see, the angle is much smaller. But the effect is different. You can not get this effect with the Peleng as you can’t shoot from inside the horse.
The same goes here, but the babyfisheye is really low-fi. I mentionned this before, If you like lo-fi you will like this one.
So my favourite still is the Peleng because it isn’t a one trick pony, it’s sharp enough to shoot more then close ups. The Diana and the babyfisheye don’t perform well on that end. But the Peleng is expensive and you need an SLR to mount it on.
Don’t read any further because I am gonna make a fool of myself. It’s a habit :)