I have owned both the Sammy 40a550 and the Sony 40v4100 for about a week. After watching both sets side by side for a week I returned the Samsung.
I had adjusting both sets the best I could, I found the Sony to have better blacks and deeper color. The Sony also could display a more detailed picture. The remote on the Sammy was sluggish to respond, the Sony was quick.
The sound on the Sony was way better the the Samsung, even after adjusting the Sammy EQ. When playing sound through the audio out, the Samsung had a horrible echo effect when the tv speakers where on, you have to turn off the internal speakers to avoid this. On the Sony you could output both at the same time with no echo effect. I know that the internal speakers are meaningless when going with an external sound system but just some fyi.
When playing video games in game mode on the Sony, you could adjust all video sliders to your pleasing. On the Samsung in game mode (entertainment mode) the video adjust sliders where locked, meaning you could not adjust anything. You where stuck with the Samsung default setting. Playing on the Samsung in a dark room in game mode (entertainment mode) was far to bright.
Both sets had no dead pixels or stuck pixels. I did not see any clouding or flash lighting on either set, if there was it was minable and equal on each set. No major smearing on either set and when there was some smearing both sets where equal.
The Sammy has a swivel base , the Sony does not. (only thing I liked better on the Sammy).
The Sony has 4 hdmi ports to the Sammys 3 hdmi ports. I have never owned a Sammy or Sony HDTV before so i have no brand loyalty.
In my opinion after owning both sets for a week I prefer Sony 40v4100 over the Sammy 40a550.