Dusk Mane Necrozma is better than Dawn wings Necrozma in most cases.
Dusk Mane Necrozma
1. Good typing defensively
2. STAB combintion is mostly unresisted in Ubers and can be covered up with EQ to round out almost perfect coverage(barring skarmory and celesteela)
3. Good boosting moves in Swords dance and Rock polish/Autotomize, thus can easily sweep with Z move or weakness policy sets. (Very good wall breaker becuz of setup options and steel typing)
4. Can even go defensive and reliably run stealth rocks.
5. Is better as a transformation to Ultra necrozma.
6. Prism Armor Lets you take many powerful supereffective attacks with proper investment.
7. Checks Xerneas(Probably the best to do so)
8. When Ultra Bursting Turns its Ground and fire weakness to a neutrality and a resistance respectively.
1. Weak to common attacking types in Ubers like P-groudon's STAB s and Yveltal's Dark attacks and prevalence of mega gengar (becoming trapped and being slower than it even after ultra burst.)
2. Scarf Yveltal and Ho-oh destroy most sets other than stone edge variants, though scarf yveltal doesn't always KO it and is threatened by Searing sunraze smash and hooh loses straight to stone edge.
Dawn Wings Necrozma
1. Ghost STAB is mostly unresisted in Ubers and can spam Moongeist Beam(Or use Its Z version to most of the time cause huge damage).
2. Other than Yveltal or any dark type in general it's STABs are amazing and can cover Yveltal with Power Gem.
3. Even though its weak to mega gengar, it is not trapped by Shadow tag due to ghost typing.
1. Yveltal's popularity in Ubers means it's mostly a liability.
2. With dark and ghost being 4X weaknesses, Prism armor doesn't mean much and dies to most(if not all) dark and ghost attacks.
3. Other than power gem for yveltal it cannot really hurt dark types in general.
4. Strictly worse than Lunala.(lunala has a better ability, can effectively run scarf, has moonblast and focus blast for dark types and ice beam for Zygarde complete)
5. Only good power boosting move is calm mind which is not very significant(Lunala does that better) and if you want to run sd, may as well run Dusk mane.
6. Is easily pursuit trapped by Marshadow(sometimes, mainly in draft league formats) and Tyranitar.(and dies to both most of the time)
And even keeping Ubers out of the picture..... Dusk mane has a better typing and is useful in most cases..... whereas Dawn wings is held back by its 4X weaknesses and you may as well use Lunala at that point.