HW is in general more competitive and academically intense, and Marlborough tends to be more collaborative. That is not to say that your daughter won’t get an amazing education at Marlborough (both do very well in college placement) or will not find supportive friends at HW, but I have seen many people leave HW because they could not handle the pressure and stress. I had the same stats as your daughter and have thrived because the competition, but it is really dependent on her personality.
I’m guessing she is the smartest in her class or just about, but she should definitely be prepared for that to change (unless she is a super-genius taking calculus in middle school). A huge shock for many new seventh graders is turning from the big fish in the small pond to the small fish in the big pond. There are many extremely smart and talented kids at both schools, but it seems more magnified and hierarchal at HW. It is almost inevitable that she will feel inadequate compared to some of the other students at some point in her HW career. Also, HW does not hold back on the seventh graders, and she should be prepared to do about 30 minutes (or 40 in honors) of homework per subject every night (it is my impression that Marlborough is a bit easier to transition into).
Marlborough has smaller grade sizes, which, depending on your daughter’s personality, may be a good or a bad thing. I have found that this makes Marlborough less cliquish than HW, and everybody seems to know everyone else. The all-girls environment also seems to eliminate much of the immature meanness that you see in the lower grades at HW, as it is mostly from the boys. They do seem to have frequent interaction with boys outside of school, however (especially with the private all-boys school, Loyola).
HW definitely over-glorifies STEM (even though they claim not to), and it will be more difficult to have a competitive GPA if your daughter is more interested in the humanities, which has been a continual source of frustration for me. This is because there is no opportunities to take honors/AP English and history until 11th grade, so the kids who had been taking honors math and science since ninth grade definitely have an advantage in college admissions. Plus, regular classes at HW are equivalent to honors classes at most public schools, so one has to be very proficient in a subject to do well in honors.
HW and Marlborough both have impressive sports programs (especially basketball and water polo at HW), but there is more focus on recruitment at HW. Overall, I will say that HW’s extracurricular activities tend to be more stellar than Marlborough. HW consistently has many nationally ranked debaters, musicians, athletes, artists, etc., and its newspaper is always in the top three in the nation. This is probably a product of having more students.
As a broad assessment, Marlborough is better socially, but HW is better academically.
Can you give me more specifics on what your daughter is interested in and what she wants out of her school experience?