The Rock Hopper and Prop Guard are not necessarily used for the same purposes.
If I’m trying to protect my prop from rocks, then yes, the rock hopper makes a lot of sense and will work much better than a prop guard.
But if I’m trying to keep trolling lines, (3-way rigs) out of my trolling motor prop, the rock hopper isn’t going to work near as well as the prop guard will.
Granted though, the prop guard in the link above is not what I’m referring to. That particular prop guard appears to be best at protecting people in the water from the blades of the prop. It’s other main objective appears to be boating performance.
Figure out what your main objective is and buy accordingly.