There have been several topics on this topic already. The trade offs in picture quality you speak of in game mode are true, due to several post process effects being removed. However, local dimming is always on and has no effect in gaming mode.
The Samsung b8500 looks perfectly fine in game mode and to play lag intensive games you have to have it on game mode or lag will be horrible. With game mode on you will not have any lag issues with this tv. The picture quality in game mode is very good. You do lose the ability to have it in "Movie" mode which gives you the most accurate picture, but its perfectly reasonable in "Standard" mode. PQ on this tv is the best you will find period. For the money it damn well should be.
The LG lh90 has very low lag due to the panel being ips and not mva like the Sony/Samsung tvs. This allows you to play lag intensive games even when your no in game mode. The lh90 is a great tv especially for the price and I would highly recommend the tv. However, the picture quality is not up to par compared to the b8500 especially in a dark room.
*Lag intensive games = fighting games, rhythm games, competitive fps.
Single player games: Uncharted, God of War, etc... seem to work perfectly fine with small input lag. Just make sure not to use high image interpolation (Judder).
What is most important to you?
Best picture quality = b8500
Best lag = lh90
Glossy screen (better pop and blacks) = b8500
Matte screen (less distraction in direct light) = lh90
Money = lh90