So, I’ve conducted tests. Here are my results:
Overlayfs consumes significantly more memory when running virtual machines, but it provides more free space for downloading files.
Copying dom0 to zram run slowly and has less free disk space, but it hardly loads the memory - I couldn’t even get the memory to 100% load, even with 17 concurrently running varlibqubes qubes!! 
Overlayfs ran out of memory after loading 10 varlibqubes qubes.
These two modes can be used simultaneously, and you can choose one of them when launching the system, depending on your needs.