Both look like they'll be utter disasters (Rhodey in the Iron Patriot armour REALLY?!?! Curdlesnoot's super flop villain instead of Mitchell REALLY?!?!), but Trek is my constant, so ST:ID. I'll probably go see Iron Man 3, too, though.
It's a tough choice, especially because I love Robert Downey Jr. as Tony Stark/Iron Man. But, when it comes down to it, as much as I'm not a huge sci-fi guy (and was never in my life a Star Trek fan) I was so blown away by J. J. Abrams's first foray into the Star Trek universe, besides the next Hobbit film, there's nothing I'm looking forward to more than Into Darkness this year.
Iron Man 3 worries me a little; it just looks like a bit of a buzzkill after The Avengers, plus I'm a little nervous that Jon Favreau isn't directing AND that it's a new writing team, different from both 1 and 2.
Star Trek. I may not be a real trekkie, because I haven't even watched the original series (sue me) but I am still more interested in Star Trek than Iron Man. I only saw The Avengers and it was not my favourite movie ever (but not bad).