If the Bills want to compete at the highest possible level next year, I believe they should trade for Alex Smith because I'm almost certain he'll play QB at a higher level than any rookie QB we can draft.
Over the past 4 years, this is the list of rookie QBs who threw more than 200 passes their first season.
Ryan Tannehill - 76.1
Brandon Weeden - 72.4
Russell Wilson - 100
Nick Foles - 79.1
Blaine Gabbert - 65.4
Christian Ponder - 70.1
Andy Dalton - 80.4
Jimmy Clausen - 58.4
Colt McCoy - 74.5
Mark Sanchez - 63
Josh Freeman - 59.8
Over the past 4 years, Alex Smith has posted QB ratings of 82, 82, 90 and 104 and has improved his YPA and TD:Int ratio every season.
Even if Smith doesn't match the numbers he put up under Harbaugh, I still think it's extremely likely that he'll finish a season under the Bills with a QB rating in the 82-89 range.
On the flip side, based on the way rookie QBs usually struggle when asked to start in year 1, I think a fair assumption is that they'll finish their rookie season with a QB Rating in the range of 69-79.
Simply put, if the Bills want to compete next year, they would be much better off trading for a vet like Alex Smith than they would be drafting and starting a rookie QB.