I’ve been spending some time comparing the Zoom MRS-8 to the HD16. I’m not familiar with either in detail so I may miss important details.
functionally it is easy to move from one to the other. They have nearly the same button and features with th HD16 having a few extra physical elements to speed up access.

I was curious if the file access (selecting projects, etc.) was faster on the HD. I assumed the SD card slowed down the MRS. Selecting projects is faster. However initial boot up is slower. Mine performs a long scan of the drive for projects.
also of interest is that the A/B loop is no smoother. Both have a delay at the end of the loop. It is nothing like smoothly looping a section in a DAW.
they are both fun to use and give their own feel to the process of creating music.