I don't think you can go wrong with either one! I love TNG and rewatch it constantly. You're right that TOS has fewer takers but I'm sure there will be some who will want to re-watch to refresh their memories on the details of the episodes. My partner and I rewatched Balance of Terror, the Cage, Arena, and Space Seed because of SNW. I'm sure others wouldn't be here if we weren't down to discuss any Trek in some way.