I recommend "all of the above".
When Spelljammer was introduced way back when, I moved them to the Astral sea and ignored the Crystal Sphere concept. They've been a part of my campaign world (although rare) for decades now, and I have a variety of different ships, propulsion, etc...
Many folks go with ships made by a confederation of dwarven smiths that have a 'moon' that orbits my equivalent of Sigil (which is also in the Astral). Those vessels work on science, not magic, whether using Astral Sails, steam jet propulsion, rockets, and the newest invention that swept the verse - rail propulsion (hollow tubes launched like a rail gun towards a target and then pulled into their goal with magic).
Others have propulsion systems based upon the old Spelljammer books, Eberron designs, Psionics, sledding (pulled by creatures of the Astral Sea that move fast), etc...
Some are built like trains, some like sailing ships, some like alien creatures, some like spaceships, etc... One legendary vessel is the body of a brain dead god.