Globally the install base gap between the two systems is almost negligent from a software perspective...in general anyways.
The gap is currently a little over 5 million if I'm not mistaken which isn't actually all that much 360 still as a general rule has higher first week sales because there is a bigger install base to sell to but the only titles that sell significantly more on the 360 are the ones that are favored by americans... because pretty much all of the console gap is in america so as a general rule games favored by americans will have a significant 360 lead while games favored by japan/others will come out either roughly equal or slightly in favor of the ps3.
It all just depends on what region the game appeals most to. As the ps3 continues to outsell the 360 consistently in others and of course japan we'll see alot more of the japan/others favored games sell more on the ps3 compared to the 360.
Of course if the 360 starts outselling the ps3 on a weekly bases in all regions and tilts its user base in others above the ps3 again then the above won't be true.
This is all just speculation and my unbiased opinion. Constructive criticism is more then welcome :)







