While the whopper is a bigger burger I think the double whopper is even better and thus I only have to order one sandwich and no fries. But that being said the whopper and double whopper have a lot more calories in them than the big mac I believe. So it would be better to eat the big mac calorie wise.
I prefer the Big Mac. It tastes better, is easier to handle and isn't over my limit of how much I can consume in one meal. I prefer the McDee's fries anyway so may as well get the Big Mac at the same time.