It is a really interesting question when you think about it!
God of war 3 will have 2 weeks on sale before NPD. GOW1 sold around 200k in 2 weeks and GOW2 sold around 650k in 2 weeks.
Final Fantasy 13 has 3 weeks to sell in march. ff games have obviously done well over the years but if we take recent titles - ff12 sold almost 900k in 3 weeks. And ff 10 sold over 1 million in 3 weeks. So because the title is now on 2 formats i expect those numbers to be higher then they would obviously if remaining a ps3 exclusive.
Battlefield Bad Company 2 in my opinion will be the game that is more critically acclaimed out of the 3 (have higher reviews %). But the previous version did not sell so great. So if we base sales on the 1st one were looking at about 500k combined.
BUT i think this game is being hugely underestimated. The beta/demo have gone down really well and everyone seems to be praising it. Some sites such as CVG going so far as to say it is far superiour to MW2. If these things can get out into the media on the same par as ffxiii and GOW3 then it could do very well.
So formats combined (360 and PS3) i would say
1. Final Fantasy 13
2. Bad Company 2
3. God of War
However if we talk about them seperatly then i would say
1. FF13 - 360
2. FF13 - PS3
3. Bad Company 2 - 360
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