You have to think about the long term people. I would choose Pam for the long term. As you can tell, Karen is the jealous type. Did you see the way she was looking at Jim and Pam when they pulled the prank on Andy? Not to mention, Pam just has a way better sense of humor. I love Karen and she's a hottie, but she's no Pam.
Now, if I had to choose one to have a romp in the hay with, Karen definitely.