I saw the 65" and thought it looked pretty exceptional during the brief interval I viewed it playing Sony's 4K demo content. It looked very good playing 1080p content, although the 1080 content I saw was animated, not a lot of fine detail, so it's difficult to say if it upscales better than the X900. The main advantage the Samsung has going is the design -- the F9000 is basically an F8000 with nicely rounded bezel edges -- and the clearer path for upgrade-ability when a new HDMI standard is released. The disadvantages would be that in the early days of 4K, the one 4K source component available (FMP-X1) simply won't work with an F9000, so you literally have zero 4K content to watch on it for the time being. It also lacks the triluminos/quantum dot of the smaller X900 models, but it's difficult to say that that is actually even a real disadvantage.
ht Panasonic 60" ZT60, Monitor Audio: Silver RX6, RX Centre, Radius 90HD; Martinlogan Dynamo 700, Marantz SR5006, PS3, Oppo BDP-103D
2ch Sony KDL-32W650A, Sony BDP-S1000ES, Marantz PM8004, JVC T-X3 tuner, Monitor Audio Silver RX1, REL T3, Apple TV, Peachtree Audio DAC•iT, Sennheiser HD598
lr Panasonic 50" ST60, Sony BDP-S5100, Apple TV
pc Energy CB-5, Audioengine D1, FiiO A1