Dear Lennert,
For new players, as a general rule Blueberry Hill Ultimate is a better local option. I play with both groups and count people in both as friends, but Rockville tends to have more experienced handlers, more disciplined defenses, and an older average age. Blueberry Hill has its share of very good players (and a number of players switch between both games), but strives to be beginner-friendly, with a younger average age (thus, it's arguably faster). Blueberry Hill Ultimate Google group here:
https://groups.google.com/g/blueberryhillultimatefrisbee
Rockville Peeps - Feel free to disagree with my characterization, but I think I've given an accurate description of the groups' differences. And hats off to our players in Rest Stop, Black Cans, and Peak Bloom, who ably represented MoCo in Colorado this weekend.