Hi, I know that the main difference is that the ax is fully modular and with a more silent fan. Despite that apparently the new HX850 is 80 Plus Gold not silver and the price difference is ridiculous (at least in Amazon).
HX850:
Amazon.com: Corsair 850 Watts 80PLUS Gold Certified Modular ATX12V/EPS12V Power Supply HX850: Computers & Accessories
AX850:
Amazon.com: Corsair Professional Series Gold 850-Watt 80 Plus Gold Certified High-Performance Power Supply - CMPSU-850AX: Computers & Accessories
Opinions? I couldn't find andy posts since the new HX850 revision