I believe the AR9485WB-EG is a single band device, meaning it's max connection speed is 150mbps. Your other adapter is most likely a dual-band which should reach around 300mbps.
There are even Wireless-N receivers that support 3x3 MiMo with 3 antenna which can reach 450mbps.
Of course I could be completely wrong as I know more about Wireless Routers than the adapters themselves.