I too think it's on an individual basis but cannot be applied to every child. I KNOW my kids are thriving with us and their birthparents were not going to do the right things for them. That's a fact. Doesn't mean it applies to everyone else though, and it doesn't mean "better off" means perfection, no loss, no grief, or "all is fine now that they have food, shelter and a wii."
Even I have some guilt at times eventhough I KNOW without a doubt that my kids are "better off". I can't play that game though because at the end of the day, it's about being the best parent I can be and the things I had no control over (prior life/history) are never going to be in my control or even "righted".