While I disagree with trading Chapman, I think he has more trade value than Bailey. And it's more than controllability.
Bailey seems to be considered a talented pitcher, capable of great things, but inconsistent. I've heard this description from people who are neutral toward the Reds as well as the media.
Chapman is viewed as a special arm and is now a two time all star. The only question I've heard about him is how he will translate to starting. He is also an attraction for fans.
I still wouldn't trade Chapman but if dealt he would likely bring more back IMO.