8m is not a massive hole in McLaren's budget, although its not insignificant.
I think taking the biggest money deal on the basis that McLaren might build a lemon and Mercedes might do a Brawn is a bit silly. I'd also like to think that F1 drivers are in the sport for more than just the pay cheque.
£8m and a chance to win the title, or £20m and a chance to fight for the tail end of the points is (should) be a decision thats about more than just money for an F1 driver.
You're assuming £8mil and a shot at the title vs £20mil and no shot, if at the end of next season Mclaren are 5th/6th, Merc are 3rd/4th people will be laughing at Hamilton for being 10-15mil worse off and further behind.
I think people are mistaking the whole brawn fluked onto a good car thing as a one off. Red Bull for me, have cheated their way through the past two seasons by doing the same thing as Brawn. In fact I don't think we've had a title winning car since Brawn started, which hasn't had some fundamental, rule bending piece of tech that gives a car a massive advantage. They finally took out most of the parts that made that Red Bull what it was, and suddenly they're missing out on Q3 in qualifying, breaking down more often(less crusing about at the front and more trying their hardest), they look visibly much slower and the lack of straight line speed with no advantage in the corners has taken them out of the equation.
So yes, for me the car that reads the rules best, or should I say, comes up with something that passes testing but is realistically against the rules, has a great shot at winning the title.
Merc aren't bad this year, put Hamilton/Alonso in the current Merc and for me they'd have more wins, qualify higher and a heck of a lot more points and that is a car that has been missing half its team and half its money for a couple years. Can Mclaren put in a full on title winning car next year, sure, if anyone in the top 4-5 teams figures out a rule bending huge advantage, it could be anyone really.
If Hamilton was at Ferrari or even Red Bull I'd say moving for money alone would be stupid, but Mclaren while a HUGE name, haven't been particularly competitive for years. Staying at Mclaren is as big a risk in my eyes as moving to Merc. He can go to Merc on a 1 or 2 year contract, make much more money and maybe have much better options in a couple years. Maybe Merc become the best car, maybe Red Bull lose a driver maybe Mclaren have the best car for the next few years. Without ANY certainty over who will have the best car next year, it seems absurd to do anything but go for the best contract you can get.
The best car next year could be any one of 5 different cars, 1 in 5 chance to be in a title capable car, 80% chance you WON'T be. I'd prefer to get myself 20mil and a 1 in 5 chance rather than 8mil and a 1 in 5 chance.