We've got one of each. Walter, our cat, was the first and we got him right after we got married ans my wife was finishing grad school and living 4 hours away. He would spend every other week with me and would ride like a champ in the car. Then right around christmas 2011 we stopped by the same humane society where we got Walter on a whim and my wife found our dog Garland. She was one of 2 pups that had survived being dumped in a ditch by someone and she was terrified of all the people there the weekend before Christmas. My wife picked her up and she just passed out in her arms, so she couldn't even put her down until we got her home. She's a weird looking idiot though, we think she's Pit Bull/ black lab or greyhound mix.
The two get along well enough, the both harass each endlessly. It's like they're both still slightly scared of each other.