Motorway is the way to go.
Its safer, quicker, causes less wear tear on the car , so it is economical in the long run. You really can't put a price tag on safety. GT road is not safe. It has a lot of commercial traffic. Too many risk factors. Tractor trollies carrying steel rods that extend way beyong the dimensions of these vehicles.
If a car needs repair on traveling on either of these roads, then its really not fit to be on these roads. Driving on old tires is not such a good idea anyway.
GT roadsuits, people who are living in one of the communities off of the GT road. There are a lot of factories on GT road, for the traffic going to these factories, it does not make a lot of sense to use the motorway. I just drove back Sialkot today. Took me 3 hours to get to Lahore, thats about how long it takes to go from Lahore to Islamabad. When i got to Lahore, I was beat. Thank God, my car has automatic transsmission, otherwise it was going to be a even worse. Its a pain to share the road with the merciless truck and bus drivers. Its not fun too see a 10 ton house traveling on your direction at a 100 miles an hour. Only God knows how many lives are wasted every year because of the bad condition of GT road.