Proven #1 WR. Definitely worth a #1 pick. With all the additional FA signings, we could afford the additional picks (plus, we had extra 5th/6th anyway from previous good moves). So it comes down to next years' 4th round pick. Still worth it IMO.
Yes, many flashy, young WR available in this years' draft, but Bills think their time to win is NOW. Don't have time for a potential #1 WR to go through the 1st/2nd year growing pains. Diggs is immediate plug-n-play.