Comparing Malgus's army to Grievous's is tuff; I'd have to really get deep to find out who would win, but I'd give a slight edge to Malgus in this since.
One-on-One? Malgus. He'd wipe the floor with Grievous. I don't usually like to say that people would win by a landslide, but I honestly think that Malgus would own Grievous. Malgus is just so beast.
I'm thinking if Malgus ever got close to Grievous then Grievous would escape. Here's a scenario: Malgus boards Grievous' flagship, but Grievous gets wind of it and escapes, activating the self destruct sequence, Malgus is pulverised!
As you can see, it doesn't have to come down a lightsaber duel, although if it did Malgus would win.