This is a tough question because it always depends on the individual cat, but I'd say that a kitten is easier. Kittens are usually pretty up for anything if they are introduced to it early -- therefore you'll only have one cat to worry about if they get behavioral. That's the only reason I say that. Otherwise a VERY mellow adult cat might be just as easy -- a cat who won't be bullied but who won't start fights either. Those are my best guesses. It will always depend on the cats' personalities, though. What is your cat like?