At first glance, the two notions seem opposite, yet they can be seen as opposite in a manner that is inherently complementary. The interpretations of the eternal return (such as those of Levinas and Derrida) that posit some sort of difference in what returns do violence to Nietzsche’s idea. They re-inscribe a Hegelian “bad infinity” into the notion of the eternal. That what returns is identical, from the metaphysical perspective, means it is not multiple, but unique. It is in this sense that, within and as an inversion of metaphysical value-thinking, Nietzsche’s eternal return is parallel to Heidegger’s non-metaphysical finitude, which is not simply our own mortality but a finitude of being itself, which as such is the unique.
What remains as not in-different, as mattering, is the self-differencing movement of finite mortality. The self-same repetition of that movement is in-different, not material to the situation. As such, precisely the “highest” value of metaphysics, intertwined with Christianity, economics (as interminable progress) and the definition of man as a dual soul-body, eternity itself, is devalued. Rather than negating it, which leaves it still covertly operative (as Nietzsche sees in atheism) it is affirmed in Nietzsche’s inversion in such a way that it is completely depreciated and thus has no value. From the quantum perspective it’s not material, i.e. whether it occurs or not, it doesn’t matter and as such is immaterial and inexperiencable. Even for Nietzsche the thought requires no truth-value attached to it, it merely needs to be thought through fully to be operative. In a parallel sense, mortality simply needs to be experienced as such for it to be operative in the sense that the finite temporalization of self-difference is all that matters, all that is not in-different to itself. Being and Time are experienced equiprimordially as what gives change and thus grants the self-difference of temporal actualization. (Or, more precisely, Being is experienced as what gives change and Time as what change is given in).
The subject of metaphysics, the apparatus of experience operative within and as metaphysical subjectivity, is thus inoperative in the eternal return of the same, since it cannot experience anything other than this particular finite return. As apparatus in the quantum sense it determines what can matter and be experienced as material. From a perspective of complementarity, Dasein in Heidegger is no longer the metaphysical subject, and as apparatus Dasein always already experiences world and itself as finite. For Dasein, there is no need to depreciate an eternity that simply isn’t. In the sense of complementarity, they are both valid, but observable via a different apparatus, that of Dasein versus the metaphysical subject.