Besides Japan where it's a PS3 exclusive, 360 sales world wide sales should be comparable with the ps3. just look at GTA4.
This is my guess.
PS3
Japan 2 million
NA 2.2 million
Europe 0.9 million
Total 5.1 million
Xbox 360
NA 2.5 million
Europe 0.7 million
Total 3.2 million
Total Lifetime Sales 8.3 Million
The reason I came up with these numbers is by using past iterations of the Final Fantasy franchise and Grand Theft Auto 4 (GTA4 was also considered a Playstation franchise by most) as references.
Of course with the Ps3 price cut and the gap between the PS3 and 360 getting closer, it could easily make my whole estimate null.