I've always liked the late shows, but the place where I've seen the late shows is where you see some comics take some big risks and do some experimental stuff. Plus, with the late shows, there isn't a rush to get done (for the next show). And, as mentioned, is for some reason "funnier".
Of course, those places were 21+. Can they even serve alcohol during the 18+ show? And I also get the feeling that the 3rd show will be full of 18 year olds. That doesn't sound like fun to me. No offense to 18 year olds, but I teach you people every day (well, 17 and 18 year olds), and I'd rather not spend more time with you.
Even though it's your preference, you always run a risk going to the latest show.
An overly-rowdy crowd can lead to an overly-irate performer. If he's spending more time dealing with drunken hecklers than he is delivering his material...it could lead to a miserable experience for you.