Correction: the being known as Cell is flawed. He can be worn, tired. Ultron needs no rest.

While Cell is organic, Ultron has various bodies, several made of adamantium. Regeneration is inferior to resisting damage in the first place. Ultron can absorb energy and can use all manner of weapons, including disintegration based attacks. Ultron even has a technorganic virus that can convert those into his servants.

Advantage; Ultron.