When I was growing up I was a DC Comics kid, probably because it was Batman who got me interested in comics in the first place. About the only Marvel title I read was Spider-Man, so I've never been much of an expert on the Marvel Universe.
So for someone like me, it was important to see "Thor" and "Captain America" before venturing into what looks like will be the blockbuster of the summer (although the Caped Crusader may have a thing or two to say about that). I found both to be solid stand-alone action flicks, but having now seen "The Avengers" (review forthcoming), it's hard to think about either as anything but the act that warms up the crowd for the headliner. But then again, I've seen some pretty good warmup acts in my time, including The English Beat for Talking Heads and Kings of Leon for Bob Dylan.
Of the two, I'd have to give the nod to "Thor" - director Kenneth Branagh brings a sense of humor to the proceedings, and the movie doesn't take itself too seriously. Chris Hemsworth is just fine as Thor, and Tom Hiddleston shows enough chops to prove that he deserved to become the bad guy in "Avengers." We can debate the reasons that Natalie Portman was in the movie, but she's certainly a lot better here than she was in any of the Star Wars prequels.
As a hero, Captain America is a bit on the dull side - not unlike Superman in the DC world, he's a bit on the earnest side (although he develops a bit of cynicism after waking up just in time to join his fellow Avengers). But Tommy Lee Jones makes a good addition as a grizzled old army veteran, and Hugo Weaving is always good for a strong villain, although his work here doesn't quite match his villainy in the "Matrix" movies.
These are not classic comic book adaptations, but they're both good and if you're only a casual fan of the Avengers universe, it's definitely a good idea to see them both before you tackle the behemoth.