I figure a rescue animal - particularly an animal that has spent some time in foster home - has already passed some threshold level of "acceptability" and is a fairly know entity. If an animal is unfit to adopt out - and there are some of all breeds who are just lost souls - chances are, that animal wouldn't be available to you. But a rescue pet is giving a wonderful animal a second (or third, or....) chance. I also believe that THEY know it and (anthropomorphizing aside) are grateful for it.

Do what feels right for you. There are so many wonderful animals in need of a loving forever home - pure breds and wonderful mutts. Maybe you'll convert more than a few to their cause.