Hi,
It all depends on what you are going to do with the Amethyst?
Tumbled and polished stones are better for using in most crystal healing aspects, they are stronger, smoother and easier to keep clean. Choosing a flat or disc shape means the crystal won't roll off so easily either. Smooth stones are good for holding and using for massage. Palm stones, thumb stones or just tumblestones can be very good for just sitting and being held or kept in a pocket. The energy given of may be diffused or mellowed byt the smoothing so that energy is emitted from the whole surface equally rather than from one tip or point.
Raw stones are more powerful and have a very strong energy emittance from the rough and jagged energies, or from the natural points and so can be used for different things. A single Amethyst crystal gives off a stronger energy than a bed or many crystal points.
So go with what you feel is right, or tell us what you want the crystal for
