I've done both successfully, but it's a crapshoot. If you have Global Entry, your chances are much better on the return trip; and if you don't have checked bags, you'll be in even better shape. Since they added kiosks at immigration in CLT, the holdup is more waiting for your checked bags than it is getting through immigration, even if you don't have GE. We get thru immigration in minutes, and by the time we get our bags, pretty much everyone else is through immigration as well. I always make sure there's a later flight in case I miss my connection.