Of course it will be better first week, there are more ps3 owners now. That's like saying they should have waited until PS3 was $199, the first week would have been more as well.
In the end, the total sale would probably be a little less on PS3 (compared to PS3 numbers if they were released together), and a little more on X360 (again, compared to X360 numbers if they were released together), because some of the people that wanted the game brought it on X360 already last year.







