850 is definitely the way to go. Plasma-like motion, great color, deeper blacks.
Also, I'm pretty sure the X-Reality Pro engine is also dual-core. As far as the MKV thing, who cares? You can use a blu-ray player or media streamer that will.
Web browser: I've used the browser on the Samsungs, and it's mediocre at best. Same with my Panasonic. Never used the Sony, but I'm sure it sucks too. Point being: who cares? You buy a TV for the picture quality, not extra gimmicks.
The 850 looks better than the 7100. That's what should matter.
ht Panasonic 60" ZT60, Monitor Audio: Silver RX6, RX Centre, RX1; Martinlogan Dynamo 700, Marantz SR5006, PS3, Oppo BDP-103D, Panamax M-5100PM
2ch Sony 32" W650A, Sonus faber Toy Monitor, REL T3, Marantz PM8004, Sony BDP-S1000ES, JVC T-X3 tuner, Apple TV, Peachtree Audio DAC•iT, Panamax MR4300
pc Monitor Audio Radius 90HD, Audioengine D1, FiiO A1
+ Sony 65" XBRX850B, Sony BDP-S5100