I think we should keep Joseph because he can do all the things we will need in a post-Manu world. Erler summed it up perfectly here:
He's right, Joseph is better at everything except off the dribble 3's, and Joseph is actually a very underrated 3 point shooter.
All else being equal I think it's a tough decision, but there's even more to it than that. We can probably get a first rounder for Mills, who is a proven playoff performer on a cheap deal for the next 2 years. In those terms it's a choice between Joseph + 1st rounder or Mills, which seems like an easy decision.