Both sets have their flaws for sure.
I owned the Samsung PN51E550 and returned it due to auto brightness adjustments during hockey, random flickering during sports and annoying buzz. I wanted to like the Samsung because overall the sports looked pretty darn good except for the Hockey issues. So after returning the Samsung, I went with the Panny GT50 and received it last Friday, the bezel is so much nicer looking compared to the Samsung, 4 HDMI slots instead of 3, 3 USB slots, SD card, WiFi and 3D (no glasses though). I have only had the set a mere 2 1/2 days and so far out of the box, blu ray movies are very nice looking, with a good amount of tweaking options but the HD sports looked grainy and washed out, I was using the standard picture mode; some folks reached out and gave me some tips so I will be putting it to the test against tonight's cup finals match.
In my opinion, go with Quality.