I think it is difficult for the rest of us to speculate as we do not have access to insider information. All we can do is speculate the best we can based on our knowledge of how F1 works and information or opinions from F1 insiders or experienced F1 people.
Based on our own knowledge of how F1 works, changing a team for a driver such as Hamilton is a very difficult decision at this moment. It might be a simpler decision in a different season or it might be even harder. But based on what we know, this current season and the way things are at the moment, it is a very difficult decision for Hamilton to make.
An F1 driver is driven to win. That is it. It is never the money or the parties or the glamour (just ask Kimi). It is that inner drive and mad ambition to just win, win every race there is to win (ask Ayrton Senna). I have had the rare opportunity before of spending some private time with Lewis Hamilton in 2007 and 2008 where I have seen first hand the mentality of an F1 driver. There is no doubt in their minds that once they sit on the grid in their cars, there is no other conclusion but winning the race. Well, at least it was with Lewis.
In order to win though you need a very fast and reliable car that can be set up for most tracks, not to mention some luck coming your way once in a while. A fast and reliable pit stop crew, a good strategist on the pitwall and lots of money to sustain the championship onslaught. It seems that this combination is readily available at McLaren at the moment and well may continue for some time to come.
Mercedes on the other hand has a reliable and somewhat fast car but it has not been the class of the field this season, not by a long shot. Rosberg and Schumacher has not really been able to challenge the top 3 teams and are actually losing steam to be challenged by the midfielders. If they are not careful they might even lose their position to Sauber who look mighty fast, reliable and can take care of its tires very well.
So Mercedes does not really have the car nor the pitstop crew to match McLaren. They do have Ross Brawn on the pitwall who masterminded Michael Schumacher's world championships and Button's. As for Money, Mercedes the car company has lots of it but whether that money will continue to flow uninterrupted to the F1 team is always a debate. The bosses at HQ are known to be not really that happy to have their millions being sucked up by F1 every year without being able to justify or quantify it at the passenger car sales level.
Seems like Mercedes has been trumped, McLaren looks like a better bet then. But then again, Mercedes does have a trump card and it is called the 2014 Technical Regulations. There will be a major change in the regulations in 2014 (as if there are not that big a chance every other year, right) where the heart of racing, the engine will be downgraded to a 1.6 liter turbocharged unit. That changes everything around it - gearbox, chassis, suspension, etc. This means that the playing field gets levelled and if Mercedes can come up with a strong package and innovation in most areas, they can steal a march on everybody else.
But then again, so can McLaren. And Red Bull. And Ferrari. So weighing all these options up, I can see that staying at McLaren makes total sense for Lewis. It's not about the money. Mercedes' offer is only marginal. It's about winning. I feel that he will stay at McLaren. Although some people seem to think otherwise, of course everybody is entitled to their opinions. But this guy seems to have the answer already as he has published it out in the open. Is he a time traveller or what?
But my opinion is based on what I know, my information could be outdated by now anyway so it could go any other way. Even Martin Brundle with all his insider knowledge couldn't make a prediction:
I guess we'll have to just wait for it.