I have 2 of each. They both have their pros and cons:
Dogs:
Pro
-Affectionate. My dogs would sit in my lap every second of the day if I let them. They like to sleep in my bed and cuddle at night.
-Generally fun to play with. You can throw sticks, make them do tricks, whatever.
-Trainable, easily.
-Barks at noises, good for guarding.
Cons-
-Big babies. They require alot of care and attention. In my household, when they are inside, they want to go outside, and when they are outside, they want to come inside. They get bored easily. If I let them have their way they'd transition between the two every 5 minutes.
-Naughty. They like to dig, chew shit up, kill every animal they come across (except my cats, but they will chase them from time to time).
-Mess up the yard with their doggy doodoo
-Eat a ton. I spend easily $80 a month on the 2 dogs.
-My dogs (in the country) are very prone to things like Lyme Disease and ticks. I have to bring them to the vet every 2 months. Both of them in the last 6 months have contracted Lyme Disease. They have had their Lyme shots, and wear frontline. Both which are very expensive. Lyme tests are $35, plus the visit fee of $25, easily $70 a vet visit. Not to mention the cost for the antibiotics, $15.
Cats:
Pro
-Depends on the cat, but can be affectionate. They like a bit more space generally, but one of my cats is a lap cat, and sits on my lap the entire time I watch tv every time.
-Trainable, but less trainable than dogs. My cats know their names and if I say it, they look at me and meow at me, so they are acknowledge me. They come when I say "kitty kitty". If I say "No", they jump off the dresser or stop annoying me.
-Eat Mice if I have them in the house. Actually they don't eat them, they bat them around and chase them around as if they were a toy. But generally I can pick up the mouse and let it outside.
-Eat and drink very little.
Cons
-Litter box, but generally that is way better than dog doodoo and easier to clean.
-Aren't very productive, or bring any value to the household other than their existance. Chasing mice maybe...
-Sometimes prone to illness and do things you don't want them to do, such as pee in your bed or in the corner.
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The end result. When comparing cost, value, etc. I'd vote cat > dog. I love my dogs very much, but they are very much pains in the ass and cost a ton of money and time compared to the cats.