It will ultimately depend entirely on preseason performance. At this point, the most obvious pecking order appears (to me) to be:
1. Cassel. Too much talent on the roster to take a risk and he's the least risky start. I think the most likely outcome is everyone is average and nobody pulls away in camp/preseason, Cassel is given the start, has moderate success, and we ride him for the entire season unless he's injured or melts down.
2. Tyrod Taylor. Rex likes him, he's mobile, and I don't see any area where he's clearly inferior to EJ. If we instal a run heavy offense with lots of motion to take advantage of Percy and Sammy...Taylor could have the most upside of any QB.
3. EJ. EJ has been bad for us in the past and proven nothing. He's mobile but not fast. Not a viable running threat by design. I fail to see how anything other than his draft spot places him above 3 on this list (at the moment)--and I don't think Bills leadership cares about where EJ was drafted based on last season.
4. Tuel. He will likely be cut and/or on the practice squad. He's a terrible QB.