The truth about cats and dogs

Five years ago my husband and I gave in to the full court press from our kids for a dog. (That's Cooper above). Now, a dog isn't enough. My son is desperate for a cat. I come from a dog family and my husband grew up without pets, so we are at a loss here. What are the chances of the dog and cat getting along? Am I a masochist for even considering it? So all you pet owners out there I'm looking for a little advice. Why do I or don't I need a cat? Are they really as self-sufficient as people say? What's the training like?