Momentum is things that may fit sports teams, and competition between one team versus another one. The reality of the marketplace is NOT momentum. People don't buy consoles due to momentum. They make choices based on personal preferences, and the preferences of those they know. They also are influenced by marketing, and library of games. But momentum? Well, that is in the heads of people who who root for a videogame console of choice.
By the way, isn't the PS3 outselling the 360?
Edit: And I just saw the 25th aniversary remake of We are the World for Haiti, and this thread about videogame companies having momentum and who is on top, and there is some sort of scoreboard seems increasingly like a vain pursuit. Isn't there more pressing things we can tend to, while using videogames as devices to distract, rather than obsess over?