I would go with BIAB for just composin tracks to play with. You can import your BIAB tunes into RealBand to mix it down and really put the finishing touches on it but it's not necessary if all you want is some rhythm tracks to play with.
It looks from the demo video that RB is much easier to enter verses, choruses, etc at certain measures on a spreadsheet table rather than copying and pasting and scrolling with BIAB.
I'm no BIAB expert but have spent some time with it.