Halo 3 to hurt PS3 sales?

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i have read a few weeks ago from two sources..game informer and gamespot that microsoft plans to release its flagship game(Halo3) during the same time as the PS3. they think that buy doing this, its gonna hurt the PS3 launch. as for me i dont quite understand that. heres why, if i have $400-500 in my pocket and if i want a PS3...im gonna go for the ps3. i probably would careless if they ship their halo3 game during the initial launch of the ps3. im just sayin how is it gonna affect ther person already planning to buy a ps3? i would understand it if that person has a ps3 and a xbox 360 and two flagship games come out during the same time...for example halo3 for the xbox 360 and lets say(im just assuming here) MSG4 or GTA. in this example i can see why now that the person who have to choose between two games if he/she had enough money to buy one game for either the ps3 or the xbox 360. i dont wanna talk sh*t on microsoft...sense im pretty sure ill be buying both systems anywayz sense i dont have no more credit cards(but thats another story..haha). i just wanna know how that is accomplished ...that a game can hurt the launch of a competitor's console. any insights on this is welcome