I'm in a similar situation, upgraded to FF and have a Tokina 11-16. I purchased a Rokinon 14 thinking that I would use it as my uwa but I'm having second thoughts, here's why...
- Tokina still works @ 16mm (no vignette, like @11-15mm)
- Tokina has AF - which I am finding to be more important to me. I didn't think so at first, but I'm finding the Rokinon hard to use (focus) except when I can stop and take my time.
- Tokina filter thread is 77mm I believe, which is the same as my 24-105, so my CPL, B&W 10 stop and other ND filters will work on both the Tok and 24-105. No getting a threaded filter on the Rokinon...
So I guess what I'm saying is that I don't know if just because you go FF that the 11-16 is obsolete, especially if you've invested in filters. Using it at 16mm might be an option (one I'm still exploring).
I should also say that I really haven't given the Rokinon a fair amount of time to try and get acclimated with it.
Good luck with your decision! I'm still trying to figure it out as well