There's a few spells which deal with one or the other, and so you might need to decide whether ethereal jaunt or astral projection works better for you. Blink and secret chest and ethereal envelope would work similarly regardless of which you choose. See invisibility (and a few similar) will work differently depending on whether your merged plane works like astral or ethereal.
There's a few feats which reference the planes, ectoplasmic spell and planar infusion being the only ones which are remotely likely to come up.
As far as I understand, regarding its 'place' or 'metaphysics', the Astral Plane is the place between the Inner and Outer Spheres, while the Ethereal Plane is within the Inner Sphere. Both of them are transitive planes.
The Astral Plane links the Inner and Outer Spheres while the Ethereal Plane links the planes in the Inner Sphere. Having a single plane that links all planes (from either Spheres) looks like a nice simplification.
Even the way they are described look really similar:
A silvery-grey void, the Astral Plane is the space between the Inner Sphere and the Outer Sphere.
This plane has very little of what most natives of Golarion would call geography. It is a place of ever-shifting mists with little hard matter to obstruct the traveler
Regarding its function, this is what I've found:
Through it pass the souls of all dead mortals on their way to their final judgement before Pharasma.
Was not able to find a function for the Ethereal Plane though, aside from being a place where there are ghosts... which are not that different than souls.
Looking at the dangers and denizens of either planes, I could not find any one that would be weird to appear in the other and there are even those that are actually common in both, like the night hags.
Regarding their planar traits, the one big difference is about time, where the Astral Plane is timeless while the Ethereal Plane have normal time.