I've played a lot of both on location, and I would choose Iron Man. Iron Man will make you a better pinball player. It's approachable and easy to understand for anyone who comes over, but difficult to master. The shots are not terribly difficult to make, but you learn serious ball control skills and recovery skills. It has enough difficult objectives to keep a novice player going for a long time.

X-Men is underrated and a fun game too. But, it's not for me. I don't find any of the shots particularly satisfying to make. The shots are tighter and more difficult to make consistently. The sound on X-Men is also a constant let down. However, if you're a house full of highly skilled players, the code is more complex. However, while I enjoy X-Men on location, I never think about it outside of when I'm there.

Iron Man keeps me thinking about playing it again and if I had only hit that ONE shot...

Of course, you've already proven that you can swap out games, so this clearly isn't a forever decision. I'd buy Iron Man, and if the fam hates it, you'll be able to switch to XMen or something else pretty easily.