Ok, gotta be like HK-47 for just a sec...
Mockery: If they were both just warped into an arena somewhere and forced to fight each other, Nihilus would most likely use that almighty 'hunger' of his to feed on Revan's power and devour him. But, if Revan wanted Nihilus removed from the galaxy, then there are surely other methods he could use to destroy Nihilus, than one on one combat. Nihilus is the more powerful entity, but Revan was the greater Sith Lord.
Adendum: Revan would win, my master was always one efficent killing machine.
(HK-47 spell over)