In the comics, yes, Phoenix/Dark Phoenix is more powerful than Onslaught I believe . . .
The episode in which Xavier is bad and turns on the X-men was lifted from a minor plot within the Phoenix Saga in the comics. No, I don't believe that was Onlsaught. I think that was just the dark side of Xavier, whereas Onslaught is a combination of both Xavier's dark repressed side in conjunction with that portion of Magneto's mind Xavier took from Magneto during the Fatal Attractions arc.