Full disclosure: total Reds homer.
That said, it partly has to do with your status in the league. If you've got a legit shot at competing for the crown, keep Wainwright. You know what you are going to get there. Chapman on the other hand could turn out to be a flame throwing mess this season. Cincy intends to stretch him out as a SP, and a little seasoning in AAA is not out of the question. Plus I'd say Chapman is as much a DL risk as Wainwright is.
So, if you're in building mode and can deal with the growing pains of Chapman, go for it. Otherwise keep Wainwright.
If you've got a second, I'd appreciate some input on my trade question...viewtopic.php?t=453223