A Masters is such a short and intense degree that a person with a Masters in that subject would most likely have more in-depth knowledge in specific areas, and but someone with an undergraduate degree would have a wider knowledge on the subject area. It's likely, however, that to gain entry onto a Masters course different to the one you did your undergraduate degree in, you would have to demonstrate that you already have some subject knowledge in the area of the Masters degree.
I really don't think it is as simplistic as one person being more qualified than the other, nor is it as easily quantifiable as this. It entirely depends on the position that you are applying for, what they want from applicants, and the career path you want to go down more generally.