I agree that it is easy to see these as the two typical romantic triangle choices (perhaps even a Betty and Veronica). Although, one could see the werewolf as the less serious one as well. After all, the Werewolf has the whole "freedom through violence, running, taking" thing. And although the vampire is the more social of them, the Werewolf is still, definitely, Hot.
What really interests me about these two, though, is the fact that they are both seeking to control their love interest. It's not just "I want to be with you"; in both cases, it is "I want you to be mine. I want to own you." I think these do keep showing up in this genre, but I'm really curious about why. I guess if the standard protagonist really is the Mundane as represented by Bella, then it's a perfect fit. That kind of character is empowered by controlling types.
I'm actually really excited about how Monsterhearts offers the opportunity to see these characters in very different circumstances. If the Mundane's lover was the Ghost, it'd be a very different story. As mentioned, I really wanna see a Werewolf and Vampire involved with one another.