My kids are in Montessori (one is in 1st, the other is 4 and in pre-k). If it's a true Montessori, then they will tailor her lessons to meet her needs. My DD4 is currently working on reading and math a year ahead of where she is and that is OK in Montessori. Given the options you have, I would leave her in Children's House/pre-school, especially if she isn't reading independently yet. They can usually accommodate up to about 2nd grade material. Even if she moved beyond that, likely they will pull things for her to do from lower elementary (if they are a good Montessori!).
The decision about formal school is hard to make until you see where she is at 5. A lot can happen in a year and a half. Does the formal Montessori start at 3 years old (most of them do). It is expensive but worth it to us. Regular public school would be a total disaster for my older child. We are looking into the full time gifted public program for him for next fall. Remember that nothing is permanent and you can change schools if warranted.
