Which should I get?

It really depends on you.
A dog is playful for longer period of its lifetime but is not able to take care of itself.
You will have to take him outside for walks even when its freezing cold outside.
As much as you might think you sure will do it, you have to really commit to it before you get one because if you dont take care of the dog, he will suffer. Dogs also require much more attention and as fun as it sounds, I wouldnt advise you to get a dog if you are young person. It also very much depends on the breed they vary very much, some need more care than others and if you are thinking of getting a bigger dog, barking might eventually get annoying since bigger dogs bark much louder.

Cats are very fun and playful at the young age but get lazier as they get older. They tend to sleep alot once they get to respectable age. Getting on a good side of a cat is very easy since they tend to stick with people that feed them most of the time. Cats can take care of themselves and letting it go outside on its own is not a problem they wont run away, they have very good memory of their path and will always comeback once it gets hungry. Cats can be fairly revengeful and if you piss off a cat, it is bound to do something bad to you. (scratch you, piss on your bed or something of sorts.)
Cats also come in different breeds and some vary in size and looks. Bigger cats tend to be more willing to explore outdoors whilst smaller breeds tend to be more indoors (with few exceptions). If you decide to let your cat roam the outside, they can be fairly competetive over the territory with other cats. So you might find your cat comeback with few scratches every now and then if your neighbours have any cats.

Please bear in mind that both dogs and cats have fur and as much as you will want the longer haired cat/dog because it is cuddly and fluffy, you will mark my word when you will find their hair everywhere... And i mean EVERYWHERE!