I use a Nikon D7100 and the most of the time my main lens is Sigma 17-50 2.8. I would like to buy in the near future a new lens more wide. I found the fisheye from Samyang 8mm f3.5 and ultra wide Tokina 11-16 f2.8. I would like to use it for architecture and landscapes. Fisheye creates nice effect because of the distortions, but Tokina has the advantage that I can attach filters. What you would recommend me? Thanks!
Really there are a few lens characteristics to consider:
How wide do you really want? 8 vs 11mm is actually a pretty noticeable difference. When you are looking at a long lens, 10-20mm is not that noticeable, but when you are wide even 1mm is certainly worth considering.
Distortion, whereas a rectilinear ultra wide angle will pull everything out and keep them relatively straight, a fisheye distorts the geometry significantly. If you want out of the camera images to more faithfully reproduce architecture you will want a UWA that is not fisheye.
The fisheye is manual focus only
Finally, you can technically convert fisheye to a UWA shot using photoshop, or software that can fix the geometry (DxO also does this). The fisheye will lose a lot of information across the image if you do this however, as you can only correct what the sensor saw so much. In my experimentation I found it slightly less trivial to convert a wide angle to fisheye using photoshop.
I'd say most of the time, the UWA is more flexible and useful than the fisheye. If you really like fisheye however the Samyangs (and Rokinson and whichever other brand for the same lens you find) is a good inexpensive lens.