The 360 definitely has had the higher attach rates from early on in its life cycle where it received the better ports of multiplatform games. That carried on through a good part of this generation until devs finally got the hang of Cell and made MP games pretty much identical in quality on both platforms. But that initial difference in platforms did cause more people to deviate away from PS3 and towards 360.
The 360 was both statistically and in perception the "core gamer" box until about 3 years ago where M$ released the Kinect and moved towards the casual audience. The PS3's initial mistakes let it capitalise for the rest of the generation and build the idea that 3rd party software is better on the 360. Hence the higher attach rates. I think that may have changed somewhat now when PS3 MP games are just as good, but the higher attach rate still remains. Plus a year with no competition should have allowed it to sell quite a bit of software.
