It does seem interesting that launch aligned the PS3 is declining quicker now. Its been a strange generation.
You're taking one outlying year and applying it for the rest of the gen. Next year, the PS3 will most likely sell on par, or better, than the 360s sales this year. It will be getting a WW cut next year, as well (or at the very least, the 12 GB will come out in all regions).
PS3 is will end up being ~16%-17% down YOY. The 360 will end up being ~24%-25% down YOY. Now for the PS3, this includes no major exclusive releases and no price cut. The only thing going in its favor was the release of the Super Slim, which actually increased the price of entry by $20 in America. For the 360, this includes a $50 price cut during America's (its best region by far) busiest shopping months, as well as the release of an installment in its most popular exclusive franchise. I'm going to guess the 360 will also be down YOY by ~25% next year, regardless of a price cut. While Sony's pricecut (first time it will be available at a constant retail price of $199), and the launch of so many quality exclusives, will help it remain only ~10%-15% down YOY.