I'm torn on this one! From a physical comfort point of view, I very much prefer sitting in the front but I can't relax mentally there - I'm either navigating or watching the traffic and trying to second guess the driver... if I'm in the backseat I can switch off and relax and just enjoy the ride!
Front seat because there is more leg room and you can see what is going on. If the radio is on, it is usually louder at the back as well. If it is a long drive, I can sometime start to feel a bit queasy at the back, but I rarely go very far in the car for it to be a problem.