So let me get this straight:
You have a friend who owns an older model 360, and it's gathering dust because I would assume he's not playing the current games available.
He was considering getting a PS3 to play what's on the PS3, but now that a slimmer 360 is out, he wants that instead? Are the 360 games that he's currently not playing going to magically get a boost in quality? Forget all the other arguments (Bluray, wifi, graphics, etc). If your friend does this, he's buying a $300 paperweight.
That's like if the Wii came out with a slimmer model right now, I'd go out and buy it just cause it's slimmer...but I don't ever play Wii games








