Ableton vs. Mainstage
IMHO, at the end of the day, Ableton is just a better product. It's a single application with two playback modes that you can use based on your performance preference. You can jump around and have a set run straight through in session view with follow actions or in arrangment mode and laying things track next to track.
Even if you're already a heavy logic user learning to use Ableton for live performance is no more, and arguably less complicated than learning Mainstage.
That's my 0.02.
I would highly recommend you check out some past livestreams on the blog or in the training center for both introduction to Ableton and Mainstage!