If the Mewtwo has 4 Energy, Melotta can trade 2 for 1.
If the Mewtwo has 0 Energy, Melotta can trade 0 for 1.
If the Mewtwo has 4 Energy, Cresselia can trade 2 for 1, taking only 120 from the revenge Mewtwo/DCE... and then KO'ing it, making the trade 4 for 2.
If the Mewtwo has 0 Energy, Cresselia can trade 2 for 0, taking only 120 from the revenge Mewtwo/DCE... and then KO'ing it, making the trade 4 for 2.
So either you trade 2 for 0 (better than Melotta), or you trade 4 for 2 (same as running two Melotta, but only takes one card).
Cresselia can also be used to easily tank a hit if you need to stall a turn, surviving easily all but two attackers in the format (Raquaza EX, and Mewtwo, if you're just stalling a hit instead of attacking), and then healing it all off on the bench.
Is it the best thing on the planet? Not sure yet. Do I want to try it? Oh yeah, very much, been excited for it for months. Is it better than Melotta? I think so.
(Note that this is all for use in Darkrai/Hydreigon, the only place Cresselia is useful. I do agree that Melotta isn't a bad choice for other decks able to supply the Psychic energy)