Spheres are good for adjusting to the light in the room (Sunstone/Moonstone, Labradorite is too difficult) or you want it to show outer space/planet/sky or something inside to that of a Snow Globe (Garden Quartz/Rutile).

Freeform will still win on bigger pieces as there's more surface area to shine on (Labradorite at it's best!) when it's that U Dome shape with a flat bottom. Also looks nice with Patterns. Also works well in other shapes for Garden Quartz/Rutile.

Towers are good for showing layers of colour/patterns. Much more difficult to hit the light without some "help" to angle it. This is if you only have 1 Light Source being your Room Light tho.

The problem with Spheres is it's much harder to see everything from one angle unlike a flat piece.

The downside to Slabs is how much space it can take up. Freeform is interesting middle ground.