Now that both LG and Samsung are shipping 55" curved OLED TVs in the US, we can expect several reviews that compare these two TVs. The first one comes from HDguru via NBC News. They like both TVs saying that they provide an image superior to any LCD or plasma TV they ever tested - but Samsung's OLED TV is better than LG's (and it's cheaper by 40%, too).
Samsung's KN55S9C was found to be brighter than the LG 55EA9800. LG's TV also suffered from dimming of test signals and stuck sub-pixels. Surprisingly, both TVs suffered from motion blur, but Samsung offers a special mode (Clear Motion) which inserts black frames between live frames. This removes the blur (but lowers the brightness).
Technically speaking, Samsung's TV is indeed superior to the LG one as it uses direct-emission OLEDs (RGB sub pixels) as opposed to WRGB (white sub-pixels with color filters).