Better legs? The 360 is still going strong and is a full year older!
Also, pretty much this entire generation after 2007 the PS3 and 360 have been the same price. The Core/Arcade cheaper 360 sku was never the big seller of the SKU's as apparent by a $280 average selling price when comparing the $199/299 models. That means 4 out of 5 360's purchased were the more expensive SKU's which were constantly set at the SAME PRICE as the PS3 SKU's. To match or even exceed the sales of your competitor when your prices are the virtually the same and your competitor even offers some additional hardware, now that's extraordinary and that's what the 360 has done this whole time.
Now, I will say if the $99 360 is just the 4GB arcade, it will have a much lesser affect than a $99 250GB 360 would. No matter what, though, it would put the sales back in line with the 360 outselling the PS3 consistently because the price points would finally be where they should have been a year or two ago. The 360 should have always maintained pricing that was drastically lower than the PS3, yet instead they matched pricing on their main SKU's the majority of their lifetimes.
In the end, it paid off for Microsoft in the form of great profits even during heavy R&D periods for their "NextBox". Where MS has made back all their money from the 360's initial losses and begun to dig into the original Xbox's losses, the PS3 still hasn't started fighting back the red ink.