Neither as cause. True migraines were onced believed to be a "vascular" headache, involving the change in vessel size as the driving force in the trigger of the ha, but in actuality it is more "neurogenic", coming from the complex symphony of nerves in the central nervous system and related nerves of the face, neck and head. The nerve dysfunction cause vascular changes and related pain.
Migrains. Probably both. First vasoconstriction and then vasodilation. The cause of many migrains is controversial and new theories of causation are developing. Stay tuned and do a lot of reading. Check out migrains via google.