Microsoft has a 7 million unit install base lead on Sony right now. All the fence sitters I know of (including myself) aren't waiting based on hardware factors, they are waiting because a. they don't have an hdtv, b. there aren't enough games they want to play on the system yet, or c. all of the above.
As for the "grandma" factor, I think that assessment is incorrect. Grandma would look at the display for Xbox and see it has more games and is cheaper and buy that if they had to make the choice themselves. Even then, I don't think that scenario represents what will usually happen in real life.
Reality : most old people wouldn't dream of trying to buy one system or another with explicitly asking the kid what they wanted. They simply don't know enough about them and don't care to learn. They want junior happy and that's about the end of it. They'll buy whatever the kids tell them to buy.