I'm just basing my assumption off of what happened with GH for PS2, where it was possible to buy just the game, without the controller, starting right around the time that GH2 came out. I don't know for sure, because the first time I ever heard of the game was at Christmas when my roommate got it. :)
Personally, I have the Xbox 360 version, and I'll be buying the one with the guitar, for precisely the reason you mentioned, the 2 player mode.
Thats a good point, I'm not sure if they are anticipating that or not, people might have to either buy another guitar (which allows for 2 player anyways) if they GH3 first then decide they want GH2 or buy GH2 and GH3 at the same time and get GH3 without the Guitar.
The thing that I find the most interesting is that there's no PS2 or Xbox 360 GH2 game only listings on the site. I haven't dug around for when they're going to be released, but they have to. It would make sense for people to be able to go in and pick up GH2 and 3 for Xbox at the same time, and all three for PS2. So there also should be a rise in the GH2 we've got listed. These games don't die when the new one comes out.
With that in mind there are a couple things to consider here:
There are already several million people who own the Guitar on the PS2 and the 360, so I would say these two are more likely to sell in larger numbers than the other versions because it costs less, the PS2 in particular.
The Wii version is also 10 dollars cheaper than other versions with the Guitar, so it's more likely to pick up new customers, on top of the game appealing to the more casual audience on the Wii.
I would say the PS2 version will likely sell about 500k over what GH2 did on it, and the 360 will probably pick up another 250-350k over GH2 sales.
On the Gamestop website, they have both with and without guitar versions listed for the PS2 and Xbox 360 versions. PS3 and Wii are only listed with the guitar, which makes sense.
Will people who already purchased the Guitar from GH2 be able to buy a copy of this game without the guitar, or does it have a completely new attachment that requires people to drop another 80-90 bucks on this?
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I'm just basing my assumption off of what happened with GH for PS2, where it was possible to buy just the game, without the controller, starting right around the time that GH2 came out. I don't know for sure, because the first time I ever heard of the game was at Christmas when my roommate got it. :)
Personally, I have the Xbox 360 version, and I'll be buying the one with the guitar, for precisely the reason you mentioned, the 2 player mode.