I have the 46" of the Sony EX500, and one of my friends have a Samsung C610 of the same size.
The picture quality is far superior on the Sony after calibration on both. The Samsung is more vivid but looks fake. The Sony shows better contrast, and despite both being 120 Hz, the Sony appears to be just slightly less blurry at fast motion. Nevertheless, blurring is rarely noticeable on either machines for regular TV programmes. A quick Googling tells that the Sony screen is also made by Samsung, but is of better quality than the one put in the C610 (different contractor).
The Samsung does have a much better remote control. My main complaint with the Sony is that its remote control feels cheap and does not have back light like the Samsung. The Sony also takes longer to start up than the Samsung. There is no clock function for the Sony.
That said, since the C610 is the step-up model of C550 and C530, I suppose it is supposed to be better than those two. And it is still my opinion that the EX500 is better than the C610 overall.
From what I know, the EX400 is the same as the EX500, but without 120 Hz. I don't think 120 Hz adds significant benefit for viewing. Actually, I turn off the Motionflow when I link my computer to the TV and when gaming, and I haven't noticed anything distracting. But I almost never watch sports on TV.