"What is the difference between dark Phoenix and White Phoenix of the crown? I know WPotC is more powerful, but by how much?"
White Phoenix is the totality of what the Phoenix is all about in its purest, complete and holistic form. Dark Phoenix is just an aspect and part of what is the true Phoenix. I am still waiting how Marvel will escape the crap they did with the White Phoenix concept though... I would have just stayed with a plain Phoenix and not deal with too much color
Anyway, I have to side with Dark Phoenix since Dr. Manhattan can only understand energy and matter...Dark Phoenix may manipulate this understanding to a level that Dr. Manhattan could not understand... I believe the Dark Phoenix has no quems in using the fullest capacity she has to win this game using her fullest psionic powers. Dr. Manhattan can not prevent future events but Dark Phoenix can and much more.