You know, at that age my girls both loved baby dolls. They loved to feed them and diaper them and "take care of them." While I don't think your daughter would be disappointed at all with a toddler doll, I would probably go with the baby--cause tiny little girls with baby dolls are just too sweet!!
I'd get her a baby now and move up to a toddler later.
And you know, there are other baby dolls out there. You could get a less expensive but good quality baby doll for her now and spend the $$ on an American Girl later.
I arise in the morning torn between a desire to improve the world and a desire to enjoy the world. This makes it hard to plan the day. ~E.B. White