No dog in this fight, but IMO they are pretty comparable. Both are regional teams (NE for Boston, the South for Atlanta) as opposed to "city teams" like KC, Baltimore etc.
Here's one site I found, granted, it's not the end al be all, but a decent indicator. You can see the Red Sox and Braves are almost identical in every category:
https://pattern.com/blog/the-most-popular-mlb-teams-on-opening-day-2021-according-to-consumer/
Also, before any brings in Social Media followers, those are very skewed as people follow multiple teams, example, as a Rays fan, I follow the Red Sox, Yankees, Jays and Orioles because we're in tha same division.