I think the main difference Is the Wifi connection speed , VR600 only 300mbps and VR900 450 mbps ( 2.4 Ghz ) , obviously you need a device that will take advantage of this extra link speed or
its pointless my Laptop has an Intel 6300 Wifi card capable of 450 mbps on both 2.4 / 5 Ghz , most devices only do 300 on 2.4 Ghz , however some manage better speeds on 5 Ghz ,
but remember 5 Ghz doesn't travel thro walls too well so if thinking 5 Ghz it really needs to be line of sight to get a fast connection link .

Check your devices that you want to connect , also I think VR600 only has 2 aerials where Vr900 has 3 , so as well as better speed the diversity of the signal with 3 aerials will be better.