Does having a PhD in epidemiology or a related study open a lot of doors?

I am curious to know if having a PhD really opens more doors in the professional world than a masters. I am uncertain where I should be investing my energy, either going back to school for a PhD or working and making my way up with a masters?

I am wanting to eventually move to a bigger city where there is more opportunity and feel that many jobs in this field require a PhD. I want to be able to be successful but feel lost as to where to start.