Agree with above. Some areas have ZERO opportunities for adults in UTR tournaments. It also has to do with your playing level, UTR tournaments are often only for levels 6+ and most adults are under that.
But it is not an either/or decision.
For UTR buying their membership is unnecessary for 99% of adult players. It doesn't get you anything other than more precise info on your opponents and more playing stats.
USTA membership is cheap, gets you the quarterly magazine and often a hat and access to enter leagues and tournaments.
All depends on what kind of competition you want to have ...