My opinion is different that pwarren; I'd learn RB first as it seems to fit your purposes better (ease of adding additional tracks). BiaB is limited in audio recording especially for additional tracks.

RB can do a lot of what BiaB does, it just does it differently (and sometimes slower). If RB seems intuitive to you after a few sessions all the better. If not, try BiaB and see if you like it better. I learned RB (PT) first before venturing into BiaB and only now do I see where it can be a real time saver, for some functions (just testing ideas and creating new song structures). But even when I do use it I move to RB eventually to add what I want and finish it off. Quite a few users move their projects from BiaB to another DAW to add tracks and finish them. So my suggestion is learn RB for your recording needs, then learn BiaB later for the time saving and 'extra' features.

BiaB does have LOTS of extra features RB doesn't. I personally don't use a lot of them and find I get the most work done in RB in the long run .. stuff I couldn't even attempt in BiaB presently..

I do not work here, but the benefits are still awesome
Make your sound your own!