My DD has been playing with AG dolls since she was 4. She really plays with them, pretty much on a daily basis. They are all still in fantastic shape. The wig brushes work really well. She is 7 now and styles their hair constantly-I can't imagine the Target dolls holding up to that kind of play.
All of my DD's dolls have straight hair, except for Molly. We don't have Nicki. I would say the easiest hair to take care of is straight and fairly short. For example, Kit, Nelli, Chrissa, Emily, and one of her non-historical dolls have hair that always looks good. A quick swip of the wig brush and they look as good as the day we got them.
DD has Felicity and Elizabeth which both have very long straight hair. When my DD was younger I used to have to sit and brush their hair out for her and I would keep both dolls in low ponytails (like the way they come) so it was easier to play with the dolls.
As a side note, I highly reccomend the DVD movies in conjunction with the dolls. When my DD was 5 and 6 years old she would sit for hours on a Saturday on my bed with either the Samantha or the Felicity DVD and play with her dolls while watching the movies. She would spread all of their clothing out and change their outfits as they did in the movie. It was the perfect thing on a winter day to keep her occupied. She now has a DVD player in her room, and this past (very raininy) weekend she was in there doing the same thing with Chrissa, her clothing, and the DVD.