If you have a decent amount of current in your tank you will want to either wedge it or tie it down because I let mine loose in my tank and the current pushed it around and knocked over a couple of my new plant starts,Just a FYI
The biggest difference I can tell you off-hand is that the Marimo balls are actually solid algae & they sink. The Java moss balls are just moss stuck to styrofoam & they float. If you go with the java balls, use some fishing line to attach it to something so it doesnt get blown around too bad.