Of your two listed choices, the Montessori sounds preferable. However, I’d urge you to ask some pointed questions of the head of schools and teachers before enrolling to build out a personalized learning plan. You don’t want your child spending 3 weeks on the letter a...
DS7 is in a Montessori elementary school, and I am certain he would have hated the Casa sequence. He was able to read at 2 like Kai’s son and thrives on conceptual discussions that extend and apply abstract thinking. Montessori, with its rigid insistence taken in many schools on concrete to abstract progression, doesn’t respect the needs of young abstract thinkers who can skip ahead of physical works.
Also be aware that, if your child is science minded, that Montessori is extremely weak in science, with focuses in language arts and math. Anticipate that you’ll have to teach science at home if you want it covered adequately.
I will say this; for older (6+) students who have good autonomy and executive function, the licence for self-directed work is difficult to achieve outside a Montessori setting. Ultimately, for HG+ students, I think the key is finding a good instructional fit; one that respects the needs of the individual learner.
