When I was 6 I got an actual American Girl Doll (kit) and I was SO happy! I think the Girls are better for a 6 year old because she will use it longer. Girls stop playing with baby dolls at 8-9 but dolls stop from 11-15(depends on the person really) and if you get bitty baby she will want you to buy her an American Girl Doll when she's like 7 or 8 and thats more money spent!
American Bitty Twins will be better than an American Girl Bitty Baby for a 6 year old girl. The twins will offer the girl variety and she can always have at least one Bitty Twin to play with in case one gets destroyed.