There are different aspects of picture quality. For pure 1080/24p video playback (blu-rays), it's hard to distinguish between players, as testified by the latest BQ. But if you're talking about DVD playback, there will be a difference between players depending on how they upscale DVD pictures.
Sound quality again depends on whether you want the player to decode audio or allow the AV receiver to decode it. If the players bitstream audio to the receiver to decode it, then there shouldn't be any difference in sound quality. However, if you allow your player to decode audio, again, there is a significant difference between players.