We are American Girl doll fanatics at our house. I have to say that the quality is NOT the same between the Our Generation and other knockoff dolls and the AG dolls. The hair on the AG dolls is so much nicer, the knockoff doll hair gets frizzy. AG doll hair stays pretty nice plus you can really comb it and style it. The quality of the clothes and accessories is just amazing. The detail!
That being said, my SD got an AG doll when she was six and she didn't take care of it. She drew on it and there are marks on the face and hands from being thrown around. You can get the limbs tightened and the doll cleaned, but it costs money.
I think if your daughters are really into the AG brand, meaning they like the dolls for their history and story then you'll want to get an AG brand. My SD likes learning about the era of the historical dolls and she likes dressing them up in their clothing. We do have some AG brand and some knockoff brand and even handmade clothing. She also takes care of her dolls now that she's older.
If your daughters just want an 18" doll to dress up then go with a knockoff brand. My SD also has an Our Generation and a Journey Girl. The quality isn't nearly as good, you can totally tell when you put the dolls side by side, but it would be fine for your young girls.
You could also start with a cheaper doll and see how they do. If they're really interested and can take care of it then maybe they can get an AG doll next year.
There is an American Girl store by us and there is NOTHING like that experience! I have to hand it to the AG brand for coming up with such great dolls, accessories and books. It's worth it to us, but to each their own. AG stuff is something my SD likes to collect, and I hope she enjoys her collection for years to come.