Your question prompts more questions than answers, mainly what purpose will the ball serve and what conditions will it be used on, but what I will say is this:
I will normally use a symmetrical ball as I can move the pin either far from or close to the axis to cut flare, depending on how rolly I want it.
Coverstock, RG and Diff chosen on the basis of the condition I intend to use it on, but lower diff balls have a lower flare potential to begin with.
I would need to know more about the intended use to help more.
