I prefer cats because they are usually able to take care of themselves to some degree and don't need hours of attention, just an occasional, "Hey man pet me. Thanks, bye."
Dogs on the other hand need to be exercised extensively, are usually obnoxious, can't live without people around, and require so much of my precious time that they don't deserve. The dogs at my house are seriously more bad than they are good. My cat does stupid and disgusting things, but nowhere near as bad as the dogs.