Plasma would give you the equivalent of 120Hz processing, so your motion for both games and TV will be designed for the reactionary picture of a video game system. The LNxxA550 units are 60hz.
Then there's the image retention or burn-in issue. If you handle your panel with care for the first 200 hours and monitor the panel using the "All white" setting (to see if you can see images in the panel), then you should be okay. But extended gaming and "RPM" indicators in driving games, say, might prove to be an issue if viewed more than 10-20% of the viewing time.
The more particular and careful you are with a plasma, the better you'll be. The fact that you took the time to ask the question indicates that you'll probably take the time and provide the minimum care and attention to keep the panel looking good.
Either way is a winner. If you're a huge gamer, and I mean 40-50% of your time will be games, then I'd recommend the A550 LCD, and maybe the A650 LCD for the 120Hz capabilities.
If you have further questions, let me know. We're here to help.
Looking for a tv for new house, to be in basement with Samsung HT-BD2E Blu Ray home theather, it will be close to window that is under a deck, so not too bright light, I will play some Xbox 360 games on it, with the price of LCD down to same as Plasma, comes down to risk of burnin, vs viewing angles vs motion blur...?
Leaning toward PN50A550 vs LN52A550 or LN46A550 (so it happens all three are on top of consumer reports ratings in their respective class)...?