Considering that the full moon barley does anything, the comet. Also that comet fire was huge.

The general enhancement of Sozin's comet is more fire, more power. Whereas the Full Moon can allow one to perform Bloodbending when they normally couldn't. But this has clearly not mattered so I would say Sozin's comet is more powerful.

Probably sozin comet as firebenders get a obviously huge boost in power while people like katara are already extremely strong without the moon so nothing different really happens besides bloodbending