I would definitely go for 32 GB RAM. Actually. I definitely DID go for 32 GB of RAM. As far as I understand the performance of the M1 that makes it interesting ist the MultiCore Performance - but Sketchup only uses one Core. (Which ist not a bad thing because you have more than one app and they can use different cores. So - Sketchup needs: One Core plus a lot of RAM… 
You need RAM.
It’s been discussed at length here: Mac Book Pro 2021 16" M1 Pro or Max - #8 by napperkt
Having said that: Sketchup doesn’t run natively (yet) and as long as that is the case there is still room for improvement. I also experience some graphical glitches and Sketchup is one of my main applications so I can’t wait for a native version. The rest of the MacBook Pro M1 still makes it a VERY worthwhile machine. Battery life, general performance, coolness of the machine, that very nice display. A semi-smooth Sketchup will come eventually. Hopefully.
I think.