As you said, neither really did much on panel. However, Mageddon had power enough to slaughter the Old Gods (whom were > New Gods) - and it took quite a "hoorah" to finally put it down. S'ivaa, on the other hand, had it taken to him by the combined efforts of Superman and Orion ().
So I'll say Mageddon, based on what we know it was used for, and what it took to beat it - compared to S'ivaa.
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"I am tired of Earth. These people.
I am tired of being caught in the tangle of their lives."