I think it's reasonable to think Seneca was better. I was at the FSU game, and his play in that game was maybe the most exciting feat I've witnessed from an ISU player. He was great, and he definitely didn't have the supporting cast or coaching that Purdy did.
But I have a hard time with the argument that Seneca was more of a "put the team on his back for a win" quarterback. Despite Seneca's talent, ISU got beaten badly in a lot of his starts, especially against good teams. That's just a fact. Brock wasn't as flashy, but his accuracy and decision making absolutely were major reasons we won so many games during his career. Many of those wins were against teams ranked much higher than us.
Like I said, I think it's a good argument both ways. I will be surprised if Purdy hangs around in the NFL as long as Wallace did, so maybe that's another mark in Wallace's favor.