I am going to Vegas in just over a week as a 20 year old. Ill let you know as I went to New york earlier in the year and have also spent much of the year in San Francisco.
New York is good fun but crazy busy, slightly overrated, but this is bound to happen as it is probably the most famous city in the world.
San Fran is good fun, but not the best city I have been to. Good fun and nice vibes, but just not my fave.