There is no difference in quality from the less expensive to the most expensive version. The basic program is identical throughout. The difference is in how many styles, Realtracks and Realdrums you will get. The more money you spend the more of them you will get.
I would suggest you buy the largest package you can afford. You will get more bang for your buck this way. Note that it is less expensive to buy the most up front than it is to buy the additional styles, Realtracks and Realdrum packages that come with the more expensive packages.
Check here for the different packages: Windows:
http://www.pgmusic.com/bbwin.packages.htmMac:
http://www.pgmusic.com/bbmac.packages.htmAlso don't worry. You have 30 days to decide if like BiaB or not because of PGMusic's 30 day money back guaranty.
After using BiaB I doubt that you will return it. Plus this is the most friendly and helpful site on the web. You will never get a Read The Freaking Manual here. There are many pros here that will help you, even if you think it is a stupid question. The only stupid question is one that is not asked.
Good luck