Assuming the same implementation effort & skill: WASM, hands down (disclaimer: I have significant RISC-V experience and am an stout advocate, but RISC-V is a poor replacement for WASM).

ADD: a few of the reasons:

- When JITing RISC-V you cannot tell which are the last use of registers without a very expensive whole-graph analysis. Without that, you will have to generate more code than necessary.

- WASM's representation is very close to what you want in the compiler. In particular, you can directly tell all incoming control-flow edges, which makes analysis much more precise, gives optimization opportunities you would otherwise miss. Recreating this information from a binary is either impossible (due to computed branches) or expensive both in terms of analysis and the guards you have to generate to protect your assumptions.

Your comment made me idly wonder if there's a way to make an ISA which lends itself both to fast (even for low-gate implementations) hardware, and to fast JIT-ing.

I suspect these are conflicting goals but an interesting question. My idle pondering are on ISAs that minimize the challenges in wide superscalar OoOE.