Well, Malgus demonstrates superior speed to be able to compete with the Republic's flashpoint team, trumps Plagueis in terms of physical strength, has far greater feats in actual combat, and his good fight against the TOR team, given how powerful the team was is above Plagueis's dueling capacity we can deduce from his Force power, speed, strength, training, etc.
Plagueis is as powerful, and I can perfectly understand people claiming Plagueis to be the more powerful Force wielder. I don't agree with it, but the reasoning is completely valid. However, any difference there is doesn't make up for Malgus being a greater fighter.