I was reading this tread and had to chime in. I have owned both of these sets... I initially bought the Sony W4100 and returned it for the Sammy B550. Why? Believe it or not the W4100 would literally give me headaches. I am not sure what about it caused the problem, but I would get a migraine within 5 mins of watching it. My suspicion is 120hz and my brain don't play nice together.
Now on the question of picture quality... the black levels on the B550 look deeper to my eyes. Overall picture quality on the Sony may have been slightly "better" out of the box, but after they were/are both tweaked it is a toss up in my opinion.
I know this has nothing to do with picture quality... but I personally like the frame, and appearance of the Sammy over the Sony. It is also considerably thinner and has a swivel stand which I use a lot more than I thought I would. The Sammy's internal speakers (B model, not A) are a marked improvement over the Sony's.
The Sony also had noticable flashlights in all 4 corners... which didn't bother me terribly, however the Sammy does not have any.