What are the benefits of getting a PhD in statistics over a MS in statistics? What are the benefits of getting a PhD in statistics over a MS in statistics (other than being a professor)? Do people with PhDs in statistics earn significantly more than people with MS degrees in statistics?

More generally, does a PhD in a quantitative field provide a salary advantage over a MS in a quantitative field?
Actually, this greatly depends on where you get your degree from. Sometimes people who have done just MS are able to get to the bottom of the thing and can use their knowledge of statistics on a practical level. It depends on how solid your concepts are. Once you are in some position earned through your sound academic record you can grow quite fast.

So, if you have good record (not just grades but grasp), you may be able to get to the same level of salary as a PhD would. This is because statistics is an applied branch and is in demand.