I too asked the same question when I was purchasing their products. The optical Rendu seemed to be the same as the ultra Rendu with a built in media converter for the +$700 bump in price. I went with the uR and decided to use two media converters to make the optical filter :
router > ethernet cat 8 > $15 switch > cat 8 > i5 Transporter.
Same switch > cat 8 > FMC > Fiber optic > FMC > cat 8 > uR > USB cable to DAC.
I had completely solved all power problems in my system to achieve a superior black background. The above still had grunge :
1. I added a LPS with a Y adapter to power both FMC = better
2. I added a LPS to the Transporter = better
3. I replaced the LPS power chords with Pangea Audio AC-14SE MkII = better
4. I wrapped my fiber optical cable in bubble wrap. It seems they are sensitive to vibrations. = better
5. I replaced the 3 post switch cat 8 ethernet cables with Pangea Premier SE = way way way better, but STILL some remaining niggly grunge!
I had not bought into the $700-$2000 ethernet switch nonsense. I had bought my $15 plastic covered switch from Home Depot. I did try a more expensive switch and sent it back because I could hear no improvement. I rewired post switch and removed the FMCs. LISTENED. Then I added the FMCs back in. They were definitely improving the sound, by a lot!
6. I ordered a iFi iPower X Ultra Low Noise AC/DC Power Supply for the switch. I took out the FMCs again and installed the new PS. WHAM!! The sound came into complete focus. I reinstalled the FMCs again and now all the grunge had disappeared. The background is black and transparent to source. I now can hear exactly what Qobuz is offering. It is easy to parse all the different sound qualities offered to us.
So, no, I have not tried the oR, but I am satisfied that I am getting the best sound and saving about $500 in the process. Of course I have 9 boxes and associated cables, but I try not to think of that.
This was a 2 year journey. I learned a ton along the way. I hope it is helpful to someone else.