A couple thoughts:
1. IMO Hogan has more upside, where Ponder has pretty much shown what he is. Hogan is really in his 3rd season of serious football (including college) and might just be turning the edge, I could see Hogan turning into a top tier possession receive this year (or he could flame out against real game day competition which is just as or more likely.) Ponder would be a decent backup, which we need. The trade probably could from a value and need argument, especially if Ponder is #3 for Minnesota.
2. It's just my opinion, but I think the Bills are wary of brining in any QB who might be considered a threat or create a QB controversy. I think it's a mistake given EJ's injuries last year, but it seems to me that they would have brought someone else in over the offseason otherwise.