Hi Eric,
Welcome to the forum.
Here's a link to the PG info page which gives a concise comparison of the two products.

http://www.pgmusic.com/salesfaq1.htm#6

Go down to #6 on the list and click.

BIAB is great for quickly composing a song and using it as a backup.

If you want to add more tracks (up to 48) and have more control over your song as in a full featured DAW then take it into Realband by either opening it up in Realband or using the Drop and Drag DAW feature in BIAB to drag all (Combo) or individual (Guitar, Drums etc) tracks into Realband.

Watching Video Tutorials for each program in the Tips Forum, on youtube and here:

http://www.pgmusic.com/videos.user.htm

will quickly give you a good idea of the features of each in action.
Enjoy,
Carkins