Korea's UBI Research posted an article comparing the picture quality of Samsung's and LG's curved OLED TVs. Overall it seems that UBI prefers LG's OLED TV as it offers better black levels and can better express the original image.
UBI says that Samsung's TV offers excellent sharpness while LG's TV has better contrast (black levels). Samsung's TV suffers from low picture quality though due to excessive gamma correction to improve brightness. As I said before, UBI says that LG's TV is better able to express the original image. Regarding viewing angles, Samsung's has excellent angles for white while LG's TV has better viewing angles for red and blue.
UBI analyzed the picture quality in three different ways: picture quality characteristics based on panel construction, picture quality characteristics according to panel and image processing technology and picture quality characteristics according to image processing.