After trying Loopback, Mobius and Sooperlooper for a Mainstage-based duo, I finally got what seems to be a solid system:
I use one instance of Sooperlooper with three tracks for the guitar player, and he controlls it with a basic FCB1010. SL takes a bit of trial and error to get the right feel for loooping, but once it's dialed in, it's great. And it has enough functions to make your head hurt.
It took forever to figure out a decent way to hook everything up, but I finally figured out that you're best off by bypassing Mainstage's MIDI mapping for SL. Use MIDI Patchbay to hook up your controller to SL and make a preset in SL for the mapping. This is the only way I could get the controller recognized every time. USB controllers seem more reliable in that sense.
I also use a few Loopback instances for bass lines and keyboard layering controlled with an APC20 Might switch that to SL, but the simpler LB functions work fine for now.