I think part of the decision would rest on whether you are using a DSLR or a film SLR.
A fisheye on a DSLR with a crop factor will give images with markedly reduced distortions at the sides because of the cropped field of view, but it's still noticeable. The images can subsequently be de-fished to produce distortion-free photos, but doing this drops the quality significantly...according to an online photo test I saw, at least.
Also, if you are using a DSLR that has a crop factor, eg. D70, D50, then the effective field-of-view on your fisheye will be not much more than that of a normal (rectilinear) 15mm lens. Not to mention that with rectilinear lenses, the fisheye sort of distortion will not be present. Images do not need to be defished in software and image quality will be better maintained.
Try to source for a WA lens for your DSLR instead. I've come to a conclusion that the Tokina 12-24mm f/4 is perhaps the best 3rd party WA available...and should be cheaper than a Nikkor 12-24mm...not to mention it's very well-built.
Check the review here for more:
http://photozone.de
Hope I got everything right. I'm not a Nikon user, so there's only so much I can say.
Additionally, the choice of fisheye or WA lens might depend on your intended purposes. If you're out for very creative images and will only use the lens once in a while for selected occasions, then the fisheye is something you can try. However, if you're primarily looking for spanning landscape shots, interior photographs or architecture, go for the WA lens. However, there is another argument that you should go for a perspective control or tilt-shift lens instead of a WA for architectural photography.
When using a WA to shoot tall buildings, the whole structure will appear to taper sharply towards the top, which makes it look very unnatural. And unless you can photograph from a good height in order to prevent this 'converging verticals' phenomenon, you'd be better off shooting with a PC lens, which you can use at ground level. Some people also use PC lenses to squeeze a bit more of whatever depth of field they have. If shifted to the correct angle, a PC lens may allow you to take a photo of a bed of flowers with all of them in sharp focus from foreground to background.
Anyway, I better let the experts do the talking here. :sweatsm: