NOTE : It wasn't mentionned in the post but I'm looking for a 55'' screen.
Budget wise, these two tvs mentionned in the title are the same or really close to each other. From different site and other references, I understood that the J6300 was better overall but as the UF6800 has 4K resolution, I'm questionning myself if at the same price, it wouldn't be a great idea to have the 4K feature as well as 120 Hz feature instead of 120 motion.
I will watch it from 12' away when in the living room and 18-20' when in my kitchen (open area). The room will have normal to a bit more natural ligthing (windows) than usual. I like to watch sports and movies more than anything else.