I've been an owner of both and I guess I like each about equally. Dogs can often be more fun and active, but they're also higher maintenance. Cats, on the other hand, are largely independent and more or less take care of themselves so long as you set out food, water, and a litterbox. I also feel like cats generally have more dynamic personalities.. they can be snarky little assholes and then become all cuddly and affectionate at the drop of a hat, whereas dogs tend to be either "ZOMG I love you, you're my best friend and I'll do anything for you!" or "I hate everybody and everything so I'm going to bark & be angry all damn day" almost all of the time. Just my experience, anyway. Really it depends on the individual animal. Some dogs & cats really suck, and some are great & very friendly.
Gun to my head and I have to pick one? Cats, then.