I have been a dog owner for over six years of my life and a cat owner for over six years of my life. Thus, I feel I am qualified to comment on the differences between cats and dogs. The truth about cats and dogs, if you will.
Anyway, here is the biggest difference between cats and dogs (besides cats being much more of a pain to take care of than dogs - a fact most would dispute but, if you've ever had to take care of both at the same time, you know is indisputable). If you are sitting down, and a cat is sitting next to you, and you get up to go to another room to get something, the cat will look at you disdainfully for disturbing its comfort but otherwise ignore you. If you are sitting down, and a dog is sitting next to you, and you get up to go to another room to get something, the dog will follow eagerly, because the dog is deathly afraid he or she is going to miss something important, probably food related, but possibly related to going outside.
And that's the biggest difference between cats and dogs.