I have heard amazing things about both of them and I certainly see myself fitting in, but they are very similar (in terms of rank and location). I can’t do revisits this year, so I’m having a very hard time making a decision.
Some information:
Just based off of this information, what school should I attend next academic year?
For the things you list, either school will serve you well, so “just based off of this information,” you’ll be fine with either one, no bad or wrong choice.
Did you get a chance to see/drive by either school on any previous visit to the US? Did you like the look and/or feel of one over the other? Is the percentage of boarders vs. day important to you because the schools differ there. Is one location better or worse for you? Do your parents have a preference?
We don’t know you or your family/situation, so we can’t help you with which might fit you better or work better for your family. For example, in hindsight, we wish our son had attended a school with an airport that he could have flown to-and-from nonstop. Logistics were tricky for him sometimes, but neither of his choices were airport-friendly for us, so it didn’t matter. For your two choices, Boston Logan is a nearby international airport, so that won’t be an issue for you. Others might want to consider travel logistics.
You have two excellent choices. Congratulations!
Thank you! Unfortunately, I didn’t get a chance to visit the campuses in person, so everything I have is based off of the virtual tours. I really hope that the virtual revisit days will paint a clearer picture.
They are both great schools!
Milton is half day student and almost twice the size of Mx I believe.
So very different campus energy I would imagine. (We never visited Milton, so I can’t say for sure.)
Milton
Middlesex
(Flip a coin)
Middlesex has an impressive 5-year college matriculation list. You might want to compare it to available information for Milton.
https://www.mxschool.edu/college-counseling/facts-and-figures/5-year-matriculation/
@merc81 , everyone in this forum has spent years explaining how prep school college lists are not a good way to pick a school.
Milton shows only 1 year results. Arguably not so different.
Back to the drawing board, OP!
College matriculation lists offer a sense of the interests and aspirations of the students at a particular preparatory school. For a student who is undecided between two schools, as is the case with the OP, they offer an accessible aspect to consider.
As ski always points out here, colleges accept students not boarding schools. It is possible to attend any college from any boarding school, and all of these boarding schools prepare students to succeed at whichever colleges they choose to attend. The matriculation lists reflect student choices, not any magic conveyed by the BS.
As a thought experiment, suppose there was a year when no Middlesex student applied to any Ivy but many Milton students did. Would you look at that year’s matriculation lists and assume that one of those boarding schools was better than the other? Or suppose six students from Middlesex went to BrassRingCollegeA but only two from Milton did. What would you deduce? Perhaps eight students were accepted to that college from Milton, but only two chose to attend, or the six from Middlesex had particular hooks or connections. You just can’t make any assumptions from these matriculation lists other than every single one of the boarding schools discussed here graduates students who are well prepared to attend any college they apply to. If it appears to anyone that any cohort of boarding schools “sends” more students to a particular college or type of college, that may well be more a reflection of the aims of the students they admit to begin with as @merc81 posted:
OTOH, if a particular college or set of colleges is your goal, I’d pick the boarding school where those colleges appear to be least popular. Less competition. 
I am a fan of the coin flip, but with a twist. Listen to that little voice in your head when it lands. If you have a little cheer or a twinge of disappointment with how it lands, follow what that voice tells you, not the coin.
If you have neither (and that has never happened to me), then go with how the flip turned out.
I just searched it up. It says that Middlesex is 250 acres and Milton is around 130. I’m not sure if it’s accurate, though.
I don’t know if it matters to you, but you may want to consider whether having a cohort of students who have been at school together in the lower school matters to you as a newcomer. Milton will, of course, have a lot of students who are new, but for some people, joining a group of "continuing " students is a negative. For many, it won’t make any difference at all.
The only thing here that leads me to differentiate is that you are international, and I assume will be a boarder? Middlesex has a bigger boarding population so I would recommend it over Milton. Other than that it’s a total toss up based on your other criteria.
I do live in the area if you want to ask questions.
I believe she was referring to student population, not campus size. And I think Mx is 350 acres, but not all is developed
Right, Middlesex has a large forest that is technically part of campus. You can get a good idea looking on google maps.
Also, while I don’t think either school has done a great job with covid, Middlesex was marginally better than Milton.
Interesting college matriculation lists–especially the 5 year matriculation list at Middlesex because it shows how many applicants were accepted & how many chose to matriculate. Helpful to see target schools & which schools are used as back-ups or safeties.
Also, as OP is interested in STEM, the college matriculation lists may help OP to get an impression of whether or not one school has more STEM oriented students.
An interesting point was made above about established friendships at Milton due to the fact that many attend from elementary school through high school while Middlesex introduces a new class with each freshman year.
Two great schools. Congratulations !
I am not sure this is that big an issue for most kids. Milton’s 8th grade is about 55 students and not all continue to 9th grade at Milton. So most 9th graders will be new. Regardless, the current 8th graders will be welcoming of the fresh blood the new 9th graders bring. So will the current 9th graders when the class expands, as it does at both Milton and Mx, for 10th grade.
Although Milton has a lower percentage of boarders relative to MX (50% vs 70%), it has a slightly larger boarding population (305 vs 285).