The topic title seems to me very misleading in the context of Buddhist practice, because nibbana is not an experience, a state or anything we can get "our minds around" so classifying it in a Western-style hierarchy of "realities" is setting up more papanca, more fallacious concepts.
Also the well-known Mayahana dictum "Samsara is Nibbana" is actually a misquote and a misunderstanding. This is what Zen teacher David Loy says about it:
Nagarjuna never actually claims, as is sometimes thought, that "samsara is nirvana." Instead, he says that no difference can be found between them. The koti (limit, boundary) of nirvana is the koti of samsara. They are two different ways of experiencing this world. Nirvana is not another realm or dimension but rather the clarity and peace that arise when our mental turmoil ends, because the objects with which we have been identifying are realized to be shunya. Things have no reality of their own that we can cling to, since they arise and pass away according to conditions. Nor can we cling to this truth. The most famous verse in the Karikas (25:24) sums this up magnificently: "Ultimate serenity is the coming-to-rest of all ways of 'taking' things, the repose of named things. No truth has been taught by a Buddha for anyone anywhere."
I guess from a conventional perspective they are worlds apart but from the ultimate view there is no essential difference because what obscures nirvana, ie ignorance and defilements are just empty and illusory habits, there is nothing substantial to them.