The answer is not very obvious, dawn wings necrozma seems a lot stronger, but it actually isn't, regular lunala is a lot faster though its special attack may be slightly lower, but it can still dish out an insane amount of damage, its ability prevents it from ever taking too much damage, and it has a slightly better move set in my opinion. Dawn wings necrozma has a lot of potential strength, its ability makes Luna's super effective moves less effective, and it has more special defense which makes it easier to survive hits, as most ghost and dark type moves are special, its move set is unfortunately, very predictable but, the main highlight of it is that you can use it in ultra burst which makes it the 5th strongest Pokemon, surpassing Arceus. So it all comes down to rather you want to use ultra necrozma along side an amazingly powerful Z-move with an amazing ability, use a very specially bulky Pokemon with a complementary item like leftovers, or a very strong bat who can survive 2 hits and dish out a lot of damage with a complementary item like life orb, some sort of gem or plate, or even possibly expert belt.
