The 360 should sell more software for the simple reason that its userbase is bigger. If I were Sony, I wouldn't worry about Forza 3. It really wasn't a big series until bundling started, and I expect Forza 3 (if it comes out this holiday season) to be outsold by Need for Speed with all platforms combined.
Microsoft said early on that they will attempt to control each holiday season by flipping between the Gears and Halo IPs. This is why we won't be seeing a Gears of War game on the PS3 any time soon, because Microsoft isn't going to give it up.
I don't know enough about Halo: ODST to make any predictions, but I'd be more worried about what the Wii will do. Even the 360's biggest hits might not look so big compared to what the Wii could do. And of course, these are all hypotheticals.
