If you're going to use either for business, I think it's just a matter of which system you prefer and--if you can clearly identify it--what your target market prefers.
In my experience, saying that Facebook is more "for family and friends" than Twitter is not very accurate. If it were so, you wouldn't see pages for clothing stores, restaurants, movie stars, etc. there. I think the reverse is equally false. Fact is, there are people on BOTH networks who use them exclusively for personal communication as well as people who use them exclusively for business/promotion. And there are plenty of people who don't really draw any distinction between personal and business. (I don't advise that, but not everyone feels that way--and that's fine, too.)
It's funny. I don't have a Twitter account, and I no longer have a Facebook page for business. But I do write tweets on behalf of one of my clients and have been approached about writing FB status updates also. So I guess you could say that even some of us who don't use these networks for business, do business through them just the same.