I have both the 203 and the 105 albeit in different systems. The 105 is in my two channel system and is my digital transport. I use the toslink to my Bryston DAC 3 for CDs and the HDMI outputs into the HDMI of the DAC for SACD, Blu Ray Audio and DVD-A. The 203 is in a multichannel system and is basically used for movies.
So to answer your question I can’t make a meaningful comparison.
With the prices that both these machines are fetching on the used market, I would go for a Magnetar. I looked long and hard at getting one before deciding to use the funds for a new OLED TV instead. The Magnetars were designed by the same team that created the Oppos and the setup menus are identical to the Oppos. I heard it at a dealer and not in my system but the movie sound and the concert disc they played really impressed me. The Magnetars weigh a lot more than the Oppos and I have to believe that some of that weight is due to superior power supplies; the rest are due to superior video processing. The UDP 800 costs the same as a used Oppo