doing what OkeyDokey said we get the following attach rates
wii PS3
overall 4.36 4.19
w/o sports/play 2.88
w/o sports 3.36
so essentially each Wii owner has as many games as a PS3 owner, except Wii sports is replacing that one game that is typically bought when the system is originally purchased.
of course, once brawl and Galaxy come out, we'll have to re-do this analysis and see how all 3 categories for the Wii skyrocketed.