Fiberglass mesh or fiberglass cloth for spackling?

For spackling, fiberglass mesh is your best bet - it adheres easily, doesn't slip under your putty knife, and holds the compound really well. Fiberglass cloth is denser and stiffer, it's harder to press into the spackling, and it's better suited for other purposes, like reinforcing drywall seams or serious repairs. If you're doing this for the first time, go with the mesh - it's easier to work with.

When applying mesh to the wall, first spread a layer of spackling, then press the mesh down and run your putty knife over it again so it embeds well in the compound. The mesh dissolves into the spackling and doesn't need extra smoothing. If there are any bumps after it dries, just sand it down with fine-grit sandpaper.

The main difference in practice: mesh gives you a neat result on smaller areas, but if you need serious reinforcement for big cracks or seams, then it makes sense to figure out how to work with cloth. But for regular cosmetic room repairs, fiberglass mesh is your choice.