850f is an IPS panel (except for maybe the 85" version--that size often uses a VA panel while the others are IPS). IPS sacrifices contrast for viewing angle, so in a dark room, blacks appear grayish, but the picture doesn't wash out as much when viewed from the side. All 900f are VA panels, which have superior contrast in a dark room, but sacrifice viewing angles.
Additionally, the 900F has local dimming, which further enhances its ability to display deep blacks. It also has a bit of an upgraded video processor, which also allows it to do Dolby Vision in addition to HDR10 (which the 850F supports), and the 900F has a slightly wider color gamut (although both have wide color gamuts). The 850F is bright enough for HDR to look good, but the 900F is one of the brighter 2018 LCD models.
The 850F, except for maybe the 85", which might be a VA panel, is best suited for a relatively bright room with wide seating arrangements. The 900F is a step up TV and is superior, although the 85", if VA, would close the gap somewhat.