Depends on how old your niece is. DD has 4 larger AG dolls (Samantha, Felicity, Elizabeth, and Emily) she also has the boy and girl Bitty's. At 8 I feel okay letting her brush and do limited styling on her big dolls. If she were 5 I wouldn't. They are such nice dolls and so expensive, they honestly require (at our house anyway) very gentle care. If I were giving her a first doll and picking from the 4 she's got I'd do Emily because she has hair long enough to style but short enough to be manageable. Also it's stick straight which is huge plus in keeping it neat. DD's friend got a JLY doll last Christmas and I saw it this summer. It looked terrible. Ratted hair and just very unkempt. So any of them *can* get awful looking if not treated well.
The Bitty doesn't have the best hair for styling but for a younger girl it would be perfect. When taken down it is a shoulder length, layered page boy cut. You can do a short ponytail, pull back the top section, do a small braid, etc.. To be honest - if she is under 8 or 9 and this is her FIRST AG doll and she hasn't got her heart set on a certain one I would go with BTs. Especially since for the price of one larger doll you can get two smaller ones (or just the girl and a couple of outfits......)
ETA - I third the wig brush, don't use anything else!