Multicore will take more time before it's stable, but it will be available via opam as another alternative OCaml toolchain until then. It's basically the result of KC's research: http://kcsrk.info/
Note that multicore includes two separate "features": the algebraic effects abstractions, and the multi-core runtime (domain heaps). From what I understand, OCaml's multi-core effort will include both of these as well as some improvements to garbage collection.
It's hard to say which is more exciting for me, but it probably comes down to effect handlers in a production-capable (if not production-ready) language that can work across different compilation target. Lots of interesting tooling that can be built on top of that!