Is there any difference? What should I start with?
Theoretically simple glass may be used, but it’s better obsidian or crystal: a quartz sphere, a black mirror, a round bottle with distilled or rose water.
An obsidian mirror is probably the best to start and then an obsidian sphere.
Crystal is to awkward as you need to throw a towel over your head to keep the light off regardless of how dark it is.
Once you get the hang of either a mirror or sphere then any object/surface can be scryed.
I Use a 100MM obsidian Sphere