If I had to pick between the two, I'd go with a cat.
It's because I don't have to take them outside and they can use the litter. They don't require a lot of food and water. If I ever had to leave town on an emergency for a few days, I would know that I can leave my cat at home without worry for a few days.
I have friends who have dogs. My friends are always arriving late and/or leaving early so they can walk their dogs. When they go out of town, they have to find someone else to take care of their dogs for them.
I've lived with 5 dogs while growing up. I remember how I have to let them out all the time and that we have to spend a lot of money to take them to a kennel while we're out of town (or we bring them with us if it's a camping trip, which is rare).
If I did have to pick a dog, then I would prefer a small dog. They're easier to care for and usually live longer than big dogs.
