I don't think Button was over-rated, at worst McLaren have made sure he isn't in another fast car taking points off them
On the face of it Lewis will show him the way home....he's faster and more aggressive.
Button fan - glass half full time; with the new regulations and potentially fragile tyres there is every chance that Jenson will qualify close to Lewis and out perfrom him in the race. Also, Lewis has shown that he still has a brain-fade problem - admittedly he has looked more mature this year, but possibly because he wasn't pressured by his team-mate and whatever he did with that car at first made him look good.
Lewis fan - Give Lewis half a sniff of a victory and he goes to a whole different level, he is very hard to handle.
Overall I think it will depend on Lewis's ability to get the best from the car over the whole race distance, perhaps his present acrobatic style won't serve him too well next year.
I'm sort of playing Devils Advocate here - personally I think that Lewis is the quicker of the two