Probs an Xbox for me - although the PS4 has got me interested if they continue to get games like The Last of Us and Shadow of the Colussus.
The 360 was a real step up from the original Xbox - the first games I got, Gears of War and Mass Effect, totally blew me away. Then I got Halo 3 and fell in love with online gaming instantly. However the past couple of years my favourite games have nearly all been single player games - Mass Effect 3 and X Com notably.
So not sure whether to go for the 360 with a great range of FPS (and Halo, which I still love) or the PS4 which is more likely to have a few quirky titles.