What is the Bu-ray plauyer that you are using?

A Sony Universal player bought on eBay for around $80! Sounds real good - and I can enjoy movies too!

Oppo BDP-105 is a great swiss army knife.  Good Blu-Ray player and also has 2 HDMI inputs and decodes audio through its DSS Sabre DACs.

Strangely, in reviews on BD players from Oppo, Pioneer and/or other brands, it's been claimed that some players have better movie sound quality than that for CD, SACD and other audio-only playback, and vice versa. 

Assuming this is true, I would not be using either Oppo for CD, SACD or any music-only purpose. Dolby Digital and DTS-MA movie sound is my only concern. And while many of my DVDs and BDs don't have stellar sounding audio, having to best hardware for those rare exceptions is a priority. https://www.blu-ray.com/movies/PJ-Blu-ray/258849/

Can anyone say whether the Oppo 203 or 105 is the better choice for movie sound?

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

I have both SONY and PANASONIC 

Pick either one with confidence for video. 
NOTE: Bluray  players are not substitutes for quality build CDPs or transports.