Have you got a more specific idea of the FX you are looking for?
For transitioning i would tend to use the pedal on the Aux Send rather than the Master Output. An integrated looper is a big plus IMO for that purpose.
I’m using the Microcosm and find it flexible enough for my needs at the moment.
I posted a similar question a few months ago, you might get some ideas from that thread: Fx pedal for the OT