buy the 400W with a dimmer. http://www.ebay.com/itm/400-watt-400w-Dimmable-HPS-MH-Grow-Light-System-Set-Kit-/150656047425?pt=US_Hydroponics&var=&hash=item2313ccd941
Both will work great. 400w will of course yield more. But depending on your setup, 400 isn't always better if you can't control the extra heat and don't have the height to keep the bulb a little further above the plants than you would with a 250w. Edit: for a 2x2 with a 400w I would highly recommend an air cooled hood if possible.
Thanks for the info guys. i should have plenty of height. About 8' or so. Im gonna have to put some kind of ventilation in. Dont think i can fit a cooled hood, but pulling cold air in shouldnt be a problem.