I think the answer to this question is what the anlytics people think.
If you look at Baseball Prospectus with framing and their new offensive stat.
Realmuto has the last 3 years has been 2.4, 4.5, 4.3 WAR player. Contreras has been 1.5, 2.5, 0.6 WAR.
Control is important, but for the Cubs the the better player is more important than control. Theo said as much when he mentioned getting a better player and giving up control. If the Cubs analytics are close to BP it really isn't a hard choice, especially with Amaya possibly being ready to play 3 years from now.