Both of these perspectives would argue that mass culture helps to maintain the status quo in a society. Functionalists would tend to argue that this is a good thing while marxists would see it as a method of oppression.
Functionalists argue that each aspect of society works to maintain the stability of the society as a whole. In this view, mass culture might be said to give a population common experiences and common frames of reference (we all care about Super Bowl ads or the Oscars or what have you). This allows us to relate to and feel solidarity with one another.
Marxists tend to see things in terms of class conflict. They would argue that mass culture would be a way of maintaining the power of the ruling classes over the workers. Mass culture would, for example, be a way of socializing people to believe in the rightness of the current system.
In this way, both see mass culture as a force in maintaining social stability, but only functionalists see this as a good thing.
I would suggest reading Marx and understand what his concept of society was at the time when he lived. From what I researched so far, I see that he was envisioning a solution to conflict and this solution is (or seemed to be at the time): abolish religion. You have to read his articles in order to see what he meant by it. You may agree with him, or you may not, but it is worth to do research in an objective manner so that you can draw your own conclusions, debate theories or refute them entirely.
At the same time, you may also do a parallel research on functionalism. You might find similarities with marxism, but you may also find things that separate the two theories.