FFXIII should outsell GOW3 first week because of the two systems and it will in lifetime sales.

It definitely will with NA/EU and both consoles for FF13.

750k PS3
350k 360

The first week sales (in Americas) of God of War II were up 500k from the first game which I believe sold 160k first week in Americas. I have a feeling that GOW III will have an even bigger first week than its predecessors and be fairly front-loaded. So, like another person said, I'd guess that GOWIII will beat FFXIII for first week sales, in Americas, but lose to it for Lifetime sales. Just by looking at the past titles. It's also important to note that the God of War collection has already sold about 600k in the Americas and hasn't even released in Others yet. If 600,000 people would go out and buy a collection of old games, I'm betting those same people are going to go out and buy God of War III when it comes out. Look out for North American pre-orders to go above 700k before release. This is my first (slightly bold) prediction on this site, so wish me luck!

ff13 will beat gow3 in both categories, first week and lifetime, and possible every single week in between.