I think Mehrunes Dagon has an entire army of immortal deadras while Molag only has petty cultist and worshippers and maybe mehrunes can instakill molag with his razor.
Not completely true. All the vampires serve Molag Bal. Not to mention one of his agent on tamriel was Mannimarco, King of Worms. Also, you encounter LOTS of his minions in ESO.
Would be better both destroy themselves as ice and fire makes steam, those Daedric princes are my favorites... I would prefer they destroy themselves, or maybe they could make a fusion: "Molag Dagon"? lol