I tried both at the same time recently! Some random thoughts about both that may or may not be helpful: 1. The USTA Flex League’s level seems to be all over the place. Not sure how they calculate it because they don’t use your pre-existing USTA rating, but I think the quality of games can vary more than UTR. By contrast, I think UTR’s matchmaking is much better, in my opinion. 2. USTA flex league requires you to install their app, but it’s actually really cohesive and pretty well-made! The only thing is since it’s an app rather than just normal texting, people probably respond slower lol. 3. USTA is cheaper than UTR, and technically gets you more matches. But honestly, sometimes people just never put in the effort to schedule their match so that aspect of it doesn’t matter much. 4. Playing in the UTR league improves your UTR rating, so it’s nice to see your progress as you get better! The USTA flex league does not improve your USTA (NTRP) rating, the only way to do that is to actually play regular season USTA leagues where you join a team.