360 wins out for me, every time. At the moment, it has a better game selection, cheaper games, better online service, better periphals, and more people own one, so its easier to find a decent game online. I can't see any of this changing over the next 12 months. Oh and its cheaper, £180 will get you an Arcade pack with a couple of games, inc one of the best driving games/sims released (IMO).
Its not perfect though..the units are built to a budget - the DVD drives don't have safety pads to secure the disks that even most basic computer drives do, I lost my 1st Forza copy to this problem (permanent fix - £2 and a couple of bits of adhesive foam). And they stop working a lot, although it seems the recent re-design of the cooling, and a switch to 65nm may sort this. The 3yr warranty for this bug is nice, but it shouldn't be necessary.