a dog is a pack (or group) animal--it really needs someone around to socialize with, depending on the breed it may require a little or a lot of excercise, big dogs eat alot and poop big, all animals need to go to the vet occaisionally and that`s never cheap. cats are much more independent; they still need love and attention, but can also usually get along with less attention and excercise than a dog might require.
with cats (hopefully kept indoors) litterboxes will be used by the cat(s) and cleaned by you at least every 2-3 days.
with a dog, you`ll be picking up warm, squishy dog poop with a plastic bag everytime you go for a walk(at least 2 good walks per day).
if you go to the pound or shelter, chances are good you can find a dog that`s already housebroken--but housebroken does not mean the animal can always hold it `til you get home, especially if you stop off for Happy Hour, or an extended shopping trip.
don`t forget to consider what happens if you need or want to leave home for a few days--who will water and feed your pet?
take it for walks? pay to put it up in a kennel while you`re gone? take pet with you? give it to ma and pa to look after?
have a friend house-sit and babysit?
dogs are fantastic companions and you will be adored by your dog if you treat it right.
cats can be very affectionate and independent at the same time--leaving home for a few days? clean the litterbox, put out a couple big bowls of water, and a big bowl of dry food, and have a nice trip!
PLEASE--really put some thought into what kind of pet you want. once you bring an animal into your life you are it`s caretaker, you are the boss, you are the one who decides what kind of life this animal enjoys.
good luck--the right pet will bring you much happiness!!