They SHOULD do both. Trade down to 2 or 3, pick up another 2nd +, and take Fergeson.
They won't do that, but they should. Like most people I'm a big Reggie Bush fan. I think he's one of the most dominant college players I've ever seen. I compare watching him play to watching clips of Lebron James play high school games.
That said, in the NFL Bush will be a good RB. That's it. And even if he is in the league of Sean Alexander, Larry Johnson, and LT (very possible), he's still just one guy. 2 out of those 3 RB's are not in the playoffs this year, and the 3rd is in the NFC.
If Buffalo were in that position, I would be screaming to trade down.