They both are fun games but get old.
Iron Man, if you aren't playing competitively and are going instead for high scores, there's really only one way to play: Set up 2x scoring, set up do or die, and then try to get 2X Do or Die. That's the only way to do it. So you'll find yourself hitting "restart" all the time if you louse it up.
Star Trek - it's a fun game but again it's repetitive...getting all the modes the second time around is basically impossible, so once you've gotten Kobyashi Maru you are again just going for the same multiballs over and over again.
Metallica is the one I'd go for if I were in the market. There's more software coming for Star Trek so the book isn't closed on that one yet.