If your not 100% sure what you want to do in life, you should take the 20 series. It is definitely harder than the 10 series. You learn a little bit more material at a slightly faster pace, but you do the most difficult problems, while in the 10 series, you'd do the normal problems. The little bit extra material you cover is usually at the end of the quarter and that material is not easy. Nonetheless, with a few more hours of studying the 20 series should be easy.
All engineering, physics, math, some econ, some chem b.s majors all require 20 series.