There seems to be much more difference between the 900E and 850E (this is the next step down), vs. the next step up, especially when factoring in the price.
I also have looked at Vizio, they're making some great tech, the new (2017) "Pro" P-series specifically, some performance metrics exceed the 900E, also has Dolby Vision in addition to HDR10 (if that's a thing), though I've read the Sony 4K sets have better upscaling vs. the Vizio, but with an ATV5, the upscaling may be occurring at the device so the TV is getting a "native" 4K input (meaning that comparison is sort of moot).
Answering your question:
OK, so all these LED sets (not talking OLED, totally different topic) are using LCD, just like an iMac, Macbook Pro, current iPhones, etc. The LED part is the backlighting (CCFL was common in older LCD sets), and how those LEDs are arranged affects the consistency and control of the lighting.
So keep that in mind for a moment.
One of the tricks used to make LED sets have better contrast, is to the vary the amount of light coming from the LEDs, but localized to specific areas (based on some kind of image processing), i.e., some are at full brightness while others may be set to lower output. Sets without full array LEDs do this, but it's less discrete, it varies the light from the sides, so the minimum area is larger, OK for some specific images, less effective for others.
Back to the full array, that's an LED arrangement where the LEDs are behind the LCD and "face forward" in a dense grid, like this:
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The more LEDs, and more "zones", the more control over specific areas of the picture. One of the sets I've been shopping has 128 discrete local dimming zones, so if you think of a 65" set, marked off into 128 squares, that's a reasonably small area, now if the picture element is way smaller, it's going to get too much light around it and have a sort of halo effect - I'm sure there's some processing logic that says, "Need more light, too small for acceptable accuracy, no change" and then you have a drop in contrast.
Helpful? More confused?
Here's a good article:
http://www.rtings.com/tv/tests/picture-quality/local-dimming