Which math series should I take as a marine bio major?

I placed into the level on the MPE where I could take either math 10A or math 20A. I am a marine bio major, and the UCSD websites generally seem to say that life science majors should just take the 10 series. I would be happy to take an easier class that doesn't require quite as much studying, and I never took calculus in high school, so I am a little nervous. However, I am concerned that I will miss out on potentially useful or necessary information if I do not take the 20 series. I also believe that the 20 series is recommended for people going into graduate school, which I am considering.

Could anyone tell me the differences between the two classes, and whether you think that it is worth taking the 20 series as a bio major?