I have both.. a Master in Economics and an MBA. I did my Master right after college just because I wanted to go to a Ph.D. in Economics, so it seemed to be the best way to get it. I would definitely recommend getting work experience and then doing an MBA, unless you want to go to the academic path.
A Master definitely helps you by signaling that you're very interested in a particular subject, and if that's what you want to do for the rest of your life (if that's your passion) go for it!
An MBA helps you to get that job on that company that you always wanted to work, by giving you access to a huge network, a lot of resources and a brand value that it will be attached to your career forever.
Basically it depends on what you want to do.