The basis of the argument is this. Both companies are upgrading an existing operating system and both claim better performance with their upgrades.
Apple has made no bones about the fact that Snow Leopard is a maintenance release designed to improve stability and integrated technologies. Microsoft is trying to get past the disaster of Vista, but the OS still the same core engine.
Overall, I think that Apple coming off its most successful release ever with Leopard with have a better upgrade than Microsoft. They are going to have to convince the consumer than Windows 7 is better than the experience they had with Vista.
That will be an uphill battle.