You have to have pass rush in the NFL - not just to deal with Mahomes. While I do not like the trade up scenario, a producing EDGE is more of a sure bet than a late rd 1 WR. Remember for years, during the drought era, the Bills seemed to neglect both lines. They need not do that again.
With all of that said, at pick 28 I would be shocked if the Bills do not take either a d-line or WR. Both are huge areas of need and you line that up with BPA.