While their numbers look similar, if you watch them play, it's pretty clear to me at least that how Barton gets his assists vs how Mills gets his assists are really different. I think Barton is a better creator.
If you aren't confident in his 3 (I am, not as much as Mills, but enough) I can see it. But I think that if you believe he can be a solid enough shooter, that getting someone who has the ability to get into the paint and create, that has some Lenten and athleticism and that can pitch in a little more on the boards is enough for me.
Plus, you get Barton on a cheap deal for one more year which is a big plus to me.