They're both great, but I don't have time for a dog. Dogs are too needy. Maybe when the kids are off to college and the house is quiet, I'll get a dog. For now, the cats will do.
Cats. I'm a quiet person and dogs are too loud for my tastes. Cats on the other hand will come to you when they want attention, and you don't have to constantly pay them attention. Give them a window, some food/water, and some kitty toys and their happy.
I have two dogs, actually one is a puppy. They do take a lot of training, but can be extremely obedient and quiet. Best of all they are loyal, kind and very comforting.
My wife also has two cats. They are nearly untrainable, demanding and annoying. I have no idea why anyone likes them.