From what I have seen so far Multiplatform titles perform better on the PS3 compared to the Xbox360 based on each console install base.
There are exception like CoD or GH ( and those exceptions are title that sell a lot negating all the gains in attach rate the PS3 made on titles selling more moderate amounts).
However first party Xbox360 titles perform a lot better than first party PS3 titles, even if you factor in the difference in the install base.
That's the reason the Xbox360 is globally ahead for attach rates....
The top 5 Xbox360 exclusives sold 26.7 million copies or so.
The top 5 PS3 exclusives sold slightly over 16 million copies.
And 16 millions isn't anywhere within 10% of 26 millions...
PS3-Xbox360 gap : 1.5 millions and going up in PS3 favor !
PS3-Wii gap : 20 millions and going down !
