I would say MK for sure, with young kids in your party. Epcot is fantastic for adults, especially with all the food options, but even my teenage brother gets a little tired of it after about half a day. He's only interested in rides, and Epcot just doesn't put thier focus in rides.
It is worth noting that MK is not only, or even mostly, just castles and princesses - great news for the boys in your life! Really the only princess oriented stuff is the castle (which is, of course, the centerpiece, I think that is why people think of it as a "Princess" park), and Fantasyland. But even in Fantasyland you have Winnie the Pooh, Peter Pan, Mickey's Philharmagic, and other things boys might like.
I think in the MK, boys would especially enjoy Frontierland, Adventureland, and Storybook Circus. Also, as somebody mentioned, Tom Sawyer Island is a good option for young kids.
Whatever you decide, I hope you have a magical first trip!