The main advantages are the bigger battery, way thinner and lighter profile(much nicer to handle), bigger screen,
and deeper blacks.
The funny thing about all this though is that for all the people whining about the build quality of Samsung phones is that if the Ativ S is anything like the Galaxy S3, it runs circles around the HTX 8 X and Nokia Lumia 920 in true build quality.
No overheating problems, reboots, creaks at the top of the phone, raised screens in one corner, power buttons that get flush after a few weeks, rattling volume rockers or other parts, light leakage through buttons and micro usb ports. Plastic yes but perfect clearance and precision engineered plastic that is tightly packed. Of course this won't go down well homers like Socialcarpet but it is true and anyone that has owned any Samsung mobile product knows it. Of course some claim they owned a GS3 and hate it but they are just full of crap usually.