There are variations in these two animals.
The Siberian cat is large, hypoallergenic, loyal, will follow you around like a dog, sit in your lap for hrs needing your companionship, alert you when someone is at the door, and some are even protective.
The Greyhound is a breed of dog that has superior intelligence, is quiet, shorthaired, clean, doesn't shed, and rarely smells. If you've ever seen a pic of one laying down looking at the camera, you'd swear he was royalty posing for a painting. This dog has a lot of dignity. Don't expect a doberman or anything.
Though these examples are extreme and not mainstream, they are domesticated breeds most anyone can attain. And you know what, I'm not even sure what my point was with that.
I think cats are really cool, but I'd have to say I personally like dogs the best. A dog is a "here and now" emotional kinda creature. His one mission in life is to make you happy. He needs you to be happy. If you're not happy he knows it and it bothers him. A cat is more reserved, to the point to where I look in it's eyes and ask myself what it could possibly be thinking. Give me the "here and now", wet nose, waggin tail and all.