a cat keeps rats away. and a dog keeps unwanted ppl away.. a cat don't tear up your shoes or stuff. a dog will destroy everything if you turn your back. cats are loving while dogs a loyal.. either one is good animals just make sure you train them not to potty or mess up your house.. "feed em love em and never leave em"
You'll get as many opinions as there are people, but I think it comes down to your personality and your lifestyle. If you have a yard, are an active person and want a pet that greets you at the door when you get home, you might like a dog better. But dogs need to go out several times a day so if you live in a city or don't have a fenced yard that means you have to go out and walk it, no matter what the weather is like. And clean up after it!
If you just want company, cats can be very restful BUT that also varies from cat to cat, and how you treat them as they grow from kitten to cat. Some cats will stay nearly as playful as kittens, and some will be lap kitties that prefer to curl up next to you (or on you!) while you read, watch TV, or try to work on your computer. I have a cat that plays fetch like a dog!
Both pets will require time and money, from the initial cost of purchase or adoption, shots and license fees, then continuing in maintenance -- cost of food, health care, treats and toys, etc. Take your time deciding, then spend as much time as you can with a potential pet to see if you seem to get along. Pets can be a wonderful thing!