The M1 and 8-16GB RAM may come into it's own further down the line as the base requirements of iPad OS get steadily notched up. Going forward iPads seem likely to receive at least 7 iOS versions, and possibly even longer security support on whatever version they get left behind on, so it's conceivable you could still be using an iPad bought today in 7,8,9 years time. If you plan on holding on to it for 5 years or more, the M1 certainly seems the better option, though if you're likely to upgrade again in 3 or less, and you really don't care about the mLED display then it might make sense to go with the 2020 if you can find it on sale at a good discount.
FWIW it sounds like the next release may be a redesign, given the talk of a glass back, though I don't really see an all glass design, probably more like a glass window in the back ~3" square to allow for wireless charging pass through.