I did a brief comparison a while back, specifically WASM vs libriscv for. I decided to go with WASM, primarily because it was much more closely designed for this sort of thing, and it works in browsers. libriscv is really cool and impressive though.

EDIT: Found this link in my notes as well: https://news.ycombinator.com/item?id=24900376

libriscv looks amazing but its's much larger. It says "Less than 40kB total memory usage". My first version (unpublished) of uvm32 was called uvm80 and emulated a Zilog Z80 CPU. My aim is to have a safe, embeddable sandbox for any device, rather than the best performance.