Honestly if somebody can simply overpower you in the Force ( which I believe DE Sidious can even to an orbalisk powered Plagueis ) then near invulnerability starts to matter a bit less. In a pure duel Plagueis would have the advantage against even a superior augmented opponent like Sidious. Since this fight will most likely come down to the Force though I'm siding with Sidious. If even one bolt of lightning gets through I'd say its enough to fry an orbalisk which would immediately release toxins into Plagueis's blood.