The AT's screw down crown is significantly taller and bigger then the GS' crown. The AT's water resistance rating is 150 m and the Grand Seiko guarantees 100 m static pressure with a non-screw down crown.
The picture is from my Grand Seiko page, do not let the different magnification of the pictures fool you. Open this page again, if you want to see the crowns' sizes in one picture showing both watches from the top view.
The GS crown is smaller but thanks to an almost resistance-free winding (I have no single other watch in my collection that winds so smoothly) the crown does not have to be big. The smaller crown turns to easy, it is a sheer pleasure to wind this watch!