When choosing a ball end mill always choose the largest size available. For the same stepover, a larger tool will leave smaller scallops, thus giving a smoother result. For a generally smooth model with some areas of fine detail, a large tool should be used for the overall job and a smaller tool should be used only to clean out detailed areas.
Larger tools cut more cleanly, have larger clearance, and stay sharp longer. The velocity of the cutting edge on a larger tool is higher for the same spindle speed.
End mills come in a variety of shapes. The most common are flat end mills and ball end mills. Flat end mills will cut flat areas with no scallops. However, they leave a terrace-like scallop on non-flat surfaces. Ball end mills will leave smaller scallops for the same stepover value on sloped surfaces, but they will also leave scallops on flat areas.
Models can be tooled with a combination of flat and ball end mills. If only one tool will beused for all surfaces a ball end geometry will give a more consistent overall feel and smooth result.
Some projects may benefit from other types of flute geometry. Contour cutting MDF or plywood sheets would benefit from down-cut spirals as the tool would push the material against the CNC machine table as it cuts rather than lift it.