The Superhero Database Classification number, or SHDB Class, is a number that represents the overall 'power' of a character. All traits of a character are used for calculating the Classification.
What it DOESN'T mean
This doesn't mean that a higher class would always beat a lower class character. But the bigger the difference in Class is, the more obvious it is who'll win in a fight.
How is this calculated
( INT^1.3 + (STR*0.5 )^2 + (SPE*0.5)^2 + DUR^1.6 + (POW + (SPS*SPL))^2 + COM^1.8 ) ^ TIER
Super Power Score and Level
Every Super Power has a score (SPS) that is used to calculate the Class. Each Super Power also has 3 levels (SPL). The level is set when connecting that Super Power to a character. The level determines the final score, of the Super Power, being used in the calculation.
Michael Demiurgos is the physical embodiment of Yahweh's infinite power, wielding the Dunamis Demiurgos, and is the creator sustainer of the Universe. As such he is close in power to The Presence.
Pre retcon Beyonder was the God of his own Universe of Beyond Realm, and had complete mastery over it, he was it rather.
Both of them are close in terms of raw power, but in terms of hierarchy PRB is above Michael, which is why I voted for him.
Can go either way though.
On the other hand, Michael Demiurge is the most powerful fallen angel of all time, and is regarded as one of the most powerful beings in the DC universe. He possesses energy manipulation abilities, reality manipulation, and telepathic and telekinetic powers. Also, he is immune to any kind of pain or injury.
However, based on the characters' deeds, I believe the Beyonder would have a slight advantage due to his ability to manipulate reality and his limitless power.