Well, this particular retailer doesn't matter in terms of UK trends, but I expect 360 to sell more than PS3 (and less than Wii) during December.
Sales have been pretty even between PS3/360 since slim, with former 360 advantage down to a slim difference most weeks, bar those with something like the exclusive MW2 bundles or the like.
However, the price cuts on the Arcade should ensure that 360 get's enough of a boost to sell more for the next few weeks. I'd love to know if MS is funding this, simply because that would make sense and it feels like a vendor funded drive to me; I can't see why the retailers themselves would be so generous about cutting their margins at this time of year otherwise.