Much better headroom with the All glass roof compared with the Sunroof! In the back seat with the Sunroof I hit my head if I don't lean it inwards towards the middle seat. With the all glass roof - no problem and better headroom. If you measure in the middle of the glass there is probably not much difference, but if you lean back your head against the headrest, the glass roof have more space for your head and also on the upper sides.
What is even more noticeable is the front seats. The headroom is WAY better. If I drive a car with the Sunroof I hit my hair in the roof and really find the bulky fabric part very annoying if I the slightest lean my head to the left. I literally hit my head in the roof. With the glass roof car there is lot's of more space and I can move around my head freely without hitting the roofliner.
In a car with the old solid metal roof, the headroom in the front is better than the Sunroof, but worse in the back. With the all glass roof you get better head space both front and rear. Clearly best of both worlds if you have long passengers.
Also note that you get a bit better headroom in new S cars with the new back seats. The seating position is a bit more concave in the outer seats so you get even more headroom.
All glass roof is EXCELLENT and def my choice if that wasn't clear at this point
. And I've really spent some time testing out the different options since I'm 6'2 with a long torso.
ps. Seats was the same in all evaluated cars. Front seat at it's lowest position and according to my seating position. Actually when I had my previous S this was one of my biggest concerns with headroom and Sunroof. Now with the all glass roof - no concerns and happy!