Karen had control and jealousy issues with Jim. After their breakup, she went off and got impregnated with a guy that looked just like him.
Ann Perkins, meanwhile, usually mimics her latest boyfriend, whether that's cowboy gear, exercising, etc. In other words, if you liked something kinky in the bedroom, she'd definitely would follow through with it. While great in the short term because you hang out all the time, the honeymoon period would abruptly switch to an area where she won't challenge you in the relationship, which will leave it stale.
My vote: threesome