Up strumming will be harder if you use a harder pick. So I recommend having a very thin pick when you learn to do the up strumming (during the last chapter of the first course).

The harder pick will be easy for picking individual string like when you play melodies.

Ideally you should have one of each to try out. I think after you learn how a good strumming sound like, you can learn to use a harder pick to strumming by turning your wrist.