Vitiate in both. Plagueis hates lightsaber combat and Vitiate never uses it, so by default it would probably be a Force-based fight even in the all-out fight. Vitiate probably wouldn't regard Plagueis as much of a threat at first, like Revan, and after being hit with a noticeable attack would overwhelm Plagueis with his superior sorcery and all around Force mastery.
Plagueis frequently engages in melee combat; he's pragmatic enough to not forgo something that would help him preserve his life just because he doesn't find it entertaining.
In what ways is Vitiate stronger in the Force? Plagueis clearly has far superior combat feats, is a more seasoned combatant, is faster, more agile, deadlier up close, etc. Even more cosmologically, his unbalancing the Force is far more impressive on a grand scale than anything Vitiate has achieved under his own power.