For me, it's not about the current speed or even what apps it may or may not run in the future. It's simply future proofing. iOS (and yes, that includes iPadOS) devices typically have very long support runs. For example, iPad Air 2 still runs iPadOS 14 and it's actually not that bad because I've been using it daily since trading in my 10.5 Pro. That's running the A8X, a supped up version of the processor in the iPhone 6. It shipped with iOS 8, and has received 6 software updates (iPadOS 14 is surely the end of the road). The last time Apple shipped an iPad with a revised SoC compared to the previous model was the iPad 3, and it was dropped at the same time the iPad 2 was: after iOS 9.
What I'm getting at: unless LiDAR is super important to you, do not expect software support on the 2020 to be longer than that of the 2018. So if you're after a really good deal, skip the 2020 and get the 2018 used or open box. The $100-$200 savings on the 2020 model over the 2021 is so negligible to me when I'm considering keeping the device for probably 5 years at this price point. You won't notice the difference now, but you sure will in another 3-5 years. As of right now, the iPad Pro 2020 is already 2.5 years old spec wise, despite only being a year old.
This also brings up an interesting discussion: now that iPad Pros have the same SoC as the Macs, how long will they each be supported? Once Apple drops Rosetta 2, I imagine they may just merge codebases and the only difference will be the interface presented to the user. iPads usually enjoy 6-7 years of support while Macs get 8-9. Could the M1 iPad be the first iPad to get 8 years of support? Maybe even longer? How much is that extra 1-2 years of support worth to you? Are you going to have it long enough to take advantage of it?