HX850: 240HZ refresh with XR 960, deeper blacks, gorilla glass, better picture all around.
HX750: 240hz refresh with XR480, no gorilla glass, worse picture/worse 3d, $500+ less.
It really depends what your price range is, both TVs share the same internet connected options for content (netflix, amazon, huluplus, DLNA, sony videos etc etc.) But the HX850 will provide better picture quality, better black levels, better 3d and faster image processing due to it's higher motionflow "XR technology".
You might be better off with an HX820, since it's a 2011 it costs A LOT less but offers the same specs (and WAY better picture quality/black levels) then the hx750.