Hey! My vote is unopppsedly for Rishikesh! The cocktail of thrill, adventure as well as serenity and peace of mind that Rishikesh has to offer is simply unparalleled! I chose Rishikesh as my last trip bachelorette trip and I was blown off. You could go for bungee jumping for an unmatched adrenaline rush. You can raft in the mighty waters of holy Ganga and experience one of the best rapids in the country. Further, in the evenings when yiu want some peace to reflect upon life, you can sit beside Ganga on any of the ghats and have a memorable time of your life! 😄👍 I stayed in camps offered by 'Animal path' at Rishikesh. If you want experience the BEST meals while in the town, do book the same. They have tents on triple occupancy. Cleanliness is average but the staff os super sweet and caring. I am actually in process of writing a blog about my Rishikesh trip, hope to complete soon 😊 Happy travelling! (ofcourse once it's safe to do so 😝)
You can do both with in 3 days... one night in mussoorie and one night in Rishikesh.
Mussoorie and rishikesh are well connected by roads and both locations are very closed with in 100km .I will suggest you first go to rishikesh then have some adventure like river rafting, bungee jumping , camping etc. and spend one night in Rishikesh. After this go to mussoorie via rishikesh - chamba - dhanaulti - mussoorie (short route as compared to rishikesh-dehradun-mussoorie).
Must visited place in mussoorie is kempty fall,George everest peak, laal tibba, gun hill point,bhatta falls etc. Best regards