Hi! I'm sorry my comment threw you for a loop, but I guess it's better to worry now than second-guess your choice after it's already made, right?
This is probably not going to be surprising advice, but I think that you have to talk to the teachers to figure out what to do. Having a teacher who truly sees your child and what he is able to do is of vital importance and is about 75% of the battle for appropriate education, in my limited experience. If the teachers in the new Montessori school "get" your son and really will allow him to work at his own (fast) pace, then it will probably be great. If not, then you're probably better off with your DS in the 3-day preschool with the teacher who understands him.
Just be SURE that the Montessori program will REALLY allow him to move at his own pace! Some Montessori schools are great about this, while others pull the bait-and-switch, promising you the moon, but then not letting the child skip the "easy" stuff. If they require lockstep completion of one task (sometimes twice!) before they allow progression to the next, it will be a long year for your DS! (It was for mine!)
So as far as the work level, I think the lockstep-ness of the school will matter more than what age the other kids are, since Montessori programs generally involve mostly independent work than traditional preschools anyway. *However*, since it can be tougher for a HG+ kid to make friends among younger ND (Normal Development) kids, that would be my one concern about the age issue. How is your DS doing with friendships among age-mates this year? Do you know if there are any other GT kids who will be in either preschool, or can you expect him to be the only GT kid? Will he be a young 4 or an old 4? Does he have interests that are like those of other kids his age, or are his interests really unusual for his age?
For example, my son has always liked things with wheels, and that allowed him to "blend" pretty well with ND kids. Sure, he knew the correct name of every piece of construction equipment when other kids called everything a "digger," and he knew all the sponsors of every NASCAR when other kids didn't know what the number on the side of the car was yet, but that didn't matter so much as it would have if he had been obsessed with, say, the stock market. The wheels helped him to fit in. Dinosaurs and superheros would function much the same way.
Does your son have interests that will help him to fit in? If not, then is he the sort of kid who will lead other kids to take an interest in his area of interest, even if it's unusual. Leadership qualities can also make it easier for an HG+ kid to be in an ND classroom, even with younger kids.
It's clear to me that good *early* communication with the school is going to be your key, I think. (You have that with the teacher you have now, it sounds like, and you'll need to build it--fast!--with the new school if you choose to go that route.) They need to know what kind of kid they're getting and you need to get more than lip service that they're going to give your son what he needs.
My one regret about our DS6's Montessori experience was that I didn't push harder to get the teachers to see him as he is. We lost basically half the year when he was 4, as they gave him only work that he could have done when he was 12-18 mos. old. Ugh. They finally caught up when DH joined forces with me to request work that were at DS's level. But it was silly that it took them so long to figure it out. I was too worried about being "that mom," but it was one case where I should have been.
I feel like I'm babbling...Does any of this help you?
