It’s actually a very interesting question. In summary, I would say that in actual terms, globally, the former (as western hegemony will undoubtedly continue to diminish) but relative to those countries who ‘remain’ in the EU, undoubtedly the latter. It is certainly unprecedented in its scale and likely long-term impact, unless you include Russia as a major developed nation, as their fall could be even greater.
A quite unbelievable reckless act of national self-harm that will make us discernibly poorer (by a number of metrics) as a country and will see the Union fracture. We have unilaterally unleashed forces that we simply cannot contain.
No turning back now. Our fate as a greatly diminished nation is sealed.