I have a cat. It's as affectionate a creature as I have ever known, but the important part is that when I am out at work or something, it's happy to do its own thing.
It's hard on a dog, I think, being left behind, they need a lot of companionship to be happy.
It's a two way thing with any animal... except for fish I guess
Also, a dog needs a good sized garder to run in, to be fair on it. Cats are a tad more adaptable.