I like them both, but the effort to reward ratio is much better for cats IMO. Don't need to train them or take them for walks, scooping a litter box takes about one minute per day if you have quality clumping litter, other clean up is limited to vacuuming some cat hair. Dogs are great but boy are they needy.
My cat was a stray who was starving out on the street when I found her, and I guess the fact that I saved her from hunger means we've bonded. She follows me all around the house and is extremely affectionate to me, but is much more apathetic to my roommate. The last time I took a trip I think that she was angry at me for leaving and pissed in the basement out side of her littler box, but it's a painted concrete floor so a bit of bleach & water cleaned that up. Otherwise no liter box incidents. Unlike a lot of other cats she also isn't picky about her food. I feed her when I leave for work and when I get back, and she quickly gobbles everything up in a minute or two. The only food problem I have is that if I fed her as much as she wanted I'd quickly have a morbidly obese cat.