Traveled a number of times, and always concluded never to take the GT Road. Here are pros and cons of both:
GT Road:
1. Road condition from Lahore to Pindi is just OK, with few pot holes here and there, but Pindi to Lahore is very very poor (at least till Mandra).
2. The risk of breaking some vehicle component is there, due to hitting a sudden pot hole. (Already happened with my friend and cousin. And mine proved tough enough to survive a few, cannot always be lucky, though)
3. Too many cities in the way, where you have to drive even in 2nd gears at times. Lot of U-turns and stupid drivers/Ghadda gari/Chingchi.
Motorway:
1. Peace of mind, less travelling time
2. Extremely bore journey, lifeless.
Fuel consumed:
My personal experience is, though Motorway distance is little longer, still fuel consumed is lesser. You continuously drive in highest gear at optimum speed.