Should I buy an XBOX 360 or wait for the PS Vita ?

I was thinking to buy an XBOX 360 S, but recently I saw the trailer for the upcoming PS Vita, so I got indecisive. What should I do ? I must mention that I am a big fan of some exclusive games like Halo and Gears of War on the 360 and Killzone and Little Big Planet on the PS3 / PSV