Re Montessori for gifted students - we've been in Montessori schools. The thing with Montessori (and really can be said of most any school) is - they are not all the same, not all teachers are the same, and not all students are motivated in the same way or enjoy the same type of learning environment. I don't think there's really any good way to know ahead of time without talking to parents who's children have attended the school, talking to the school staff, taking a school tour, and going with your gut feeling.
I could not agree with this more. We have been in Montessori schools from the beginning and have had both awful and wonderful teachers (even at the same school!). Good luck!