HN user
Assuming your username on HN is the same across social media platforms, you should make it easier for folks to contact you. i.e. do not require folks to know your email address to reach out via LinkedIn. It may be annoying to get random invites, but you never know where interesting opportunities might come from.
Congratulations!
There really is no bad answer here. Regardless of which you choose, you can always cross register at the other institution. MIT/Harvard students are allowed to enroll in classes at the other institution, space permitting (Tufts, too if that is of interest). The schedules for classes do not line up perfectly, but if there are specific classes you really want to take at the other institution, you can make it work.
I was a student at Sloan and took three classes at HBS. In my experience, the Harvard students were very welcoming and the elitist stereotype did not prove true. When HBS students enrolled in my Sloan courses, they also proved to be friendly teammates.
I enjoyed my experiences at both institutions and don't feel you can go wrong here. If the network is the main thing you are after, HBS has a larger network as the class size is more than 2x the size at Sloan. Good luck with your decision, it seems like a great problem to have.