I think much depends on what your main use for the car will be. Having had both, there is significantly more room in the back of the QP, as you would expect. It is a proper 4 door saloon with all that entails. That said, the GT is incredibly spacious in the back for a coupe and offers real world flexibility. I have taken my wife and two daughters to the south of France and to Italy on driving holidays and there is enough luggage space to pack for 2 weeks, just! The GT is also a much nicer car to drive if you want to press on or just cane it when on your own. The QP is actually very impressive for what it is but ultimately it is a much bigger and heavier car than the GT and it shows.
If you are looking for a car that can occasionally carry 4 people or do the odd long trip with the kids, the GT may well suit but if you need the space and comfort of a proper 4 door on a regular basis, then the QP is likely to be the one to go for. Either way you cannot really go wrong, both are great cars. I would however advise going for the 4.7 version in both, makes a huge difference to the way the cars drive, the 4.2l really doesn't have enough torque low down, especially in the QP which is not a light car! Good luck.