Ever since I earned my first acheivement point on my 360 I find it hard to play games without them. I seriouly won't even consider a PS3 or Wii port of a game even if it was a little cheaper. I don't have a PS3 but i think they have something with the throphies or something like that but 360 got me first and now my view on playing games has completely changes. It's just one of those little things that enhances the experience just enough to sway my vote towards a 360 version over anything else. Especially with buying new games. If the game costs the same on both the PS3 and 360, it just seems like you're getting just a little more for your money with just a simple little bit of something that really means nothing.
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I forgot to mention the part where I'm really not looking forward to getting a PS3 mainly because I would only be buying it for exclusives and first party games and exclusives are a dieing breed and there's only so much any first party publisher can do to support a system. Even Nintendo can only go so far to make a system worth buying and the 360 with just that small little idea of acheivement points won me over big time.