Hi Josh,
With Audiobus 3 you can assign Midi bindings to the mixer controls - that would be how to switch between them - though I can’t think how to switch at the end of the loop.
What exactly are you wanting to do.? Are you singing and playing at the same time and sometimes looping the guitar, sometimes looping the voice, and sometimes looping both?
If you were only ever looping either guitar or voice you could assign them both to the same midi binding note/cc and then set one in Audiobus to MUtE the other Unmuted. Then when you press the MIDi trigger it will switch between them.
If you want a more complex switching system I think you will need to use AUM as it allows much more control for specifying how the MIDI cc values affect the result. I think the only downside is that it seems only one MIdI biding can be set for each Control - whereas in Audiobus you can set multiple.
The most complete setup would be a midi controller that can send multiple midi cc / notes on one button press with AUM set up to respond to what you want to do.
The whole system can get quite fiddly depending on your needs.