Key differences between the two 65” models fall on Active 3D vs. Passive 3D, of which the more expensive 900A is Passive rather than Active. The 900A also has more powerful audio output and side-positioned front-facing speakers (65 watt) rather than the down firing speakers on the 850A. If weight is a concern the higher tier 900A weighs about 17lbs more than the 850A.
However, in terms of video quality both displays are impressive, providing up to 3840 x 2160 resolution at 24p, 25p and 30p, 4096 x 2160 resolution at 24p, MPEG Noise Reduction, 3D support (2k only), 24p True Cinema, 4K X-Reality Picture PRO processing, Motionflow XR 960 technology, Dynamic Edge LED backlight, and a 64.5″ viewing area.
The Sony Store currently has both 65” models in stock if want to check it out. Look for the TVs tagged “Sale” from among the selections.