I prefer dogs atm, as I have never owned a cat.
The best thing about dogs is their loyalty, even if you treat them badly sometimes, they still love you. Yes, their are downsides(all the barking) but having dogs has been great for me.
Even in moments of deep depression, dogs cheer people up. It always puts a smile on my face when I go and sit outside and my dog jumps up onto the chair next to me.
At times the only good things in my life were my dogs. They cheered me up, and are always showing affection. No matter what mood you're in, a dog can adapt. Whether you want to play or be still, a dog will always be there, by your side.
When I was 5, we got our first dog. I was horrible. I would pick it up and drop it, from about chest hight, and wack it for no reason. One day, I was stung by a bee. I went and sat down in the corner of our garden, bawling my eyes out, only to find my dog walking up to me. Licking my finger, she managed to move the stinger to a posistion where I could get it out.
She remains loyal to me to this very day, even though she still flinched if I move my hand too fast and is afraid of heights.