What is the major difference between these two 46" TVs?

Well, the biggest difference between Samsung's 8000 and 8500 series is the type of LED lighting used. The 8000 uses LED edge type lighting. The 8500 uses true LED back light with local dimming. The Samsung 8000 series all use edge lighting LEDs, which produce a fairly uniform/consistent light level across the entire picture. The Samsung 8500 series uses back lighting LEDs (in groups), which can be dimmed in groups or individually, to help with contrast. In most viewing, the 8500 series produced a little bit better contrast than the 8000 series. So I think 8500 series is still better than the other.