Hi Noel,
Main stage is for live performance - you can create layers, splits, choose patches etc etc and switch from one performance to another. Main Stage uses Logic.

Logic is a full DAW with a wonderful collection of instruments and synths. You cannot do what you can with BIAB - i.e. add chords and create an arrangement. You can play guitars and other instruments with an iPad connection live choosing chords as you go along and strumming, finger picking etc.

I use BIAB - Mac version which I thing is fantastic - and transfer the various tracks into Logic - so easy. You can use Garageband instead of Logic for this and this is equally as good.

Hope that answers your questions.

Best Regards
Jim