FFXIII and Gow 3 are selling considerably better than their predecessors, but, this mean they are getting better lifetime sales or are they just becoming more front-loaded?

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This. But my guess is that they really are expanded their userbase. I expect to legs for FFXIII be stronger at 360 than at ps3, since 360 owners are for who the series wants to grow. For GoW 3 is just impossible to know, maybe little markets are pushing sales too, in Brazil there was like 100 people in front of Sony Style waiting to buy the games. Of course, many others store were selling the game, it should have sold near 10.000 copies here, a good number for us.

god of war 3 is going to have quite the word of mouth... anyone that has played it (with small exception as always) loved it... so it can have legs...

final fantasy 13 on the other hand... is going to front loaded... the fans will buy it no matter what... but the word of mouth may not be that of god of war.