Nic Batum is more of a spot-up 3 point threat vs Rudy Gay who is more of a mid-range/high post kind of guy.
Maybe by looking at it, Rudy is more of a ball stopper while Nic would definitely fit better in a pick and roll situation, being a legitimate 3 point threat.
Rudy, however, can fit that go to guy especially since he performs well in the clutch.
Perhaps the last argument merits the difference in contracts .... ? It's subject to opinion.
I would be happy with either one, with each one bringing something different. I just like the idea of Rudy Gay as the first player since who knows when that you can put the ball in the end and say, "get me something."