Someone on an f1 website reminded me of Schumacher's move to Ferrari in 1996. The team was incredibly inconsistent for many years. Now, today's Hamilton has not shown the leadership or strength of Schumacher circa 1995 but maybe that's what's he'd be aiming for with a move. Or maybe he just wants a change and more control over his image (more personal sponsorship, more money.
Also, with the 2014 engine changes, there's expected to be a big advantage for teams with works engine deals. Mercedes, Ferrari and Renault lead team (presumably Red Bull) would be the ideal places to be in that case.
I don't see why Mercedes will have much of an advantage with the engine. Mercedes will provide the whole powertrain to McLaren, and then the car will be designed around it. Ferrari has shown no particular benefit from having everything in-house since they lost the advantage of unlimited testing at their own track. Red Bull's advantage is Adrian Newey, not being the lead Renault team.
Lewis should stay at McLaren. It could work out at Mercedes, but there's nothing to suggest it will.