'Has anyone been to both Amorgos and Astipalea? If so, and you could only choose one, which one and why? How many days/nights is appropriate? Thanks!'
I have been to Astypalaea a few times and stayed in Amorgos, both ports, more often, but the practicalities have always intervened as to how long I stay on Astypalaea or whether I actually get there. The main difference is in the amount of tourists, Astypalaea makes Amorgos seem touristy. Amorgos has more tangible 'sights' if that floats your boat. I really can't write a guide book, but why not simply go and see and experience for yourself, that's what Greek Island hopping is about.
There is only about 3 ferries a week between Amorgos and Astypalaea so that can set how long you stay. I have always found ferries unreliable to/from Astypalaia, sometimes being left with a once weekly service, there is a link with Kos but one time I was there this conked out as well.