Sanding or spackling - what helps against cracks in the ceiling?

The crack needs to be cleaned first before you apply filler - if there's still dust or loose pieces in there, the compound won't hold properly. After that, apply the filler, let it dry, and then sand it for a smooth surface.

You need to spackle first, yeah, but watch out: if the crack keeps coming back, it's usually because of movement in the substrate or poor adhesion - then even the best spackle won't hold permanently if you don't work with a primer first and prep the crack properly.

Before you even start spackling or sanding, I'd take a closer look at the crack - is it a fine hairline crack or something bigger? That makes a difference, because with larger cracks the cause matters, like the others already mentioned. If the crack keeps coming back, even the best spackle won't help you, you really have to first figure out whether the ceiling is moving or if there's a structural problem. But with a normal, stable crack you do it like I said: clean it, put in some thin spackle, let it dry, then if needed spackle again if it's sunk in, and finally sand it for a smooth surface.