[Veselin Kolev]

In my dictionary, a hopper is a gravity powered shoot that lets only one ball drop through at a time. A magazine is a mechanism that does not rely on gravity to do this. With a magazine, you can control load rate, etc...

It also depends on where the "cannon" is on your robot. If it is on top, you probably can't use a hopper to load it (as gravity is a downward force... usually...) so you would have to use a powered magazine. Another reason you may want to use a magazine is to ultimately prevent balls from jamming in your robot, you just have a single row magazine that makes the balls form a single file line.

However if your cannon is at the base of your robot, a hopper is safe to use, as long as you design one that can't easily jam. And hoppers are a lot easier to make.

EDIT: you can have gravity powered magazines. It's just a magazine that is flipped upside down. No kidding, gravity can be useful... this is new.