Well, the 17" uses a larger screen, and I don't think they've gotten around to making those LED (in mass quantities?) yet. The advantage would be that the LEDs don't contain mercury in their backlights. The smaller screen and less power consuming LEDs do make the 15" more energy efficient and therefore have a longer battery life.
PS, if you're looking for a MBP, I'm selling mine.
Well at lot of people have been having trouble with the LED screens, see older threads on yellow screens, screens with severe gradients etc.
So I'm totally fine if they don't get LED screens into the next 17" MacBook Pro because that's the one I want, sans crappy screen issues on a top price piece of kit.