I have dated both. I find that anorexia is usually a more difficult situation to deal with in general, but since I like to help people, it's not something that I've ever shied away from. And I like curves, so I've never had a problem with someone being slightly overweight either. If their personality is compatible, that would be enough.
I think it depends on the guy's/girl's character and personality. If i met a great and wonderful guy and i'm happy with him i would date him no matter if he's anorexic or slightly overweight, we could ask him to change couldn't we?