I think as with most things, which iPad Pro is better depends on your situation and how you use it. For my situation, which is likely not typical, the 12.9" iPad Pro is better.
This is what went through my thought process:
1. I saw the wonders of having an external monitor with my coworkers. Then I wanted to use Sidecar
2. I was using the 2017 iPad Pro 10.5" - it still runs just fine so I didn't really feel a strong need to upgrade, but the screen is too small for my taste as a second monitor
3. my mom was using the original iPad Air from 2013 I gave her, running the A7 processor. It's slow and also not going past iOS 12.5.5
4. I saw starting from 2018, the iPad Pro has USB-C, and so does the iPad Air 4. That was an important feature to me in my next iPad. I wanted to be able to do HDMI out easily, and I have USB-C hubs for that purpose
5. I considered the iPad Air 4 or 5, but eventually decided the 10.9" was still too small for an external monitor. The ability to use my next iPad purchase as an acceptable external monitor became the main factor, with USB-C also being important. So iPad Pro 12.9" from before 2018 were out for me.
I ended up getting a used 2020 iPad Pro 12.9", and gave the 2017 iPad Pro 10.5" to my mom. I get an iPad Pro upgrade plus a good sized portable second monitor with Sidecar, and my mom gets an iPad that's not slow, has a bigger screen, and gets the latest OS updates.
I will say that I am disappointed with the reliability of Sidecar when running wirelessly, so I connect it to my Mac with the USB-C cable when I use that feature.
Thinking back, if my only reason for the purchase was to have an external monitor, then I should have gotten a portable 15-17" monitor. But with addition of giving my mom a better iPad, I think I'm ok with the choice I made in end, even though it's a bit pricey.