They are actually remarkably similar.
- Edge lit system has the lights along one or more edges, with filters on the front to create dimming zones.
- FALD systems use the same filters, but the difference is that the LEDs are directly behind the screen and arranged to create more zones.
From a technical stand point, with test patterns etc, this creates a much higher dynamic contrast.
In real world use however, this isn't the case, because the filters are the same; if you have a bright element next to a dark section, the filter will still not be enough to block the light bleed from the bright section...as you saw on the pictures in the Sony thread.
The difference seems bigger on paper due to the technical test data and seems better in anecdotes due to what I like to call "buyers bias". "Buyers bias" is a placebo whereby you think something is performing better than it is or better than a competing model, because you paid more money for it, etc, etc.
The issue with newer TVs, HDR or not, is NOT how dark they go...but rather that they get far brighter than old TVs. Dimming systems were created to try and get around this issue, but are a very imperfect system regardless of which is implemented. Because neither is capable of effectively countering the heightened brightness of newer TVs; and this is getting worse as more and more TVs try to get brighter to provide better HDR.