If you're looking for the full experience and advantage of an MBA, I would suggest waiting a couple of years. You might have work experience from working while you're in school, but it's not of the caliber you might think it is. I don't mean that to demean your experiences but merely to point out you will be going toe to toe with some of the brightest from around the world. Not to mention, the additional experience will let you get SO much more out of your education.
As for the masters in Marketing - marketing is a tough field in the first place. A masters right out of undergrad may not give you the boost you're looking for. Once again, you will earn a return on what you bring to the table.
You're only 20 - you have time. Schools want people to bring diverse experiences to the table not only for their own benefit, but yours too. Take a few years, get some additional experience, and explore! Don't rush into an MBA. Not to mention, at 20, almost all of the top MBA programs will raise their eyebrows (and not in a good way).
I hope this helps and good luck!
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