Steam games are so bloody convenient and I like having access to all the community integration. Even if I just use Steam to share screenshots for most games I still appreciate having it so easy to do. The other thing I loved was not having to sign in twice after launching the game with Steam because it was allowed to remember my account information.
This is no brainer... Steam is easiest way to get nice boost of new players. They just need to pay to get it on Steam's front page for a day or so (as on the large head banner like some big releases), and people will flock in thousands.
I just hope there will be enough servers to handle the wave of new players. Nothing turns off a new player more than sever queues >.<
Throwing it up on steam is a no brainer. You get large amounts of people due to the wide availability of the platform, and the sales ensure that even more people will come to join the game as well as that there is an uptick of money during the big spending seasons.