honestly, comparing a manual diaphram manual focus cheap fisheye to a good ultra-wide angle lens is really apple versus orange.
these kinds of fisheyes are good fun for certain instances, and can be used creatively with good effect.
but as for day to day wide-angle shooting, a nice rectilinear wide angle is much more convenient. you get full metering, AF, don't have to defish to get straight lines.
its my opinion you can't replace a good ultrawide with a fisheye, rather think about augmenting your arsenal with the fisheye when you want to get creative