I have to say if it where just Grievous vs. Malgus mano e mano... My credits are on Malgus, Obi wan made swift work of him and I feel Malgus's offensive darkside/sith abilities would definatly give him the edge, as Grievous has no such abilities or affinity with the Force with which to defend himself.

If we take armies into account I think it'd be a close fight, and am going to go with Malgus as the victor, as I would like to think the human element would win out over a droid army and ofcourse he has Sith within his ranks, easily worth a good lot of battle droids...