I like both,
A dog, ofc you will have alot of fun with it, its gonna be your best friend for a while, and will always stay in your heart even after. But you have to take care of it every day, like its your child, and not everyone always have the time for that.
A cat, is realy easy to take care of it. You feed him, he sleeps. They barely play, and when they do, they play alone. But its always fun to have somebody to pet and all, to feel less lonely. And when they doe, ofc you will be sad, but you get another one and thats it.
For now, i have a dog, im young, i have plenty of time to take care of him, but maybe in few years, when hes not gonna be there anymore, and i will live alone, i will get a cat.