Powerheads direct current in a single, uniformed direction. They are mostly meant for filters (internal, overhead, canisters).
Wavemakers on the other hand creates waves as the name suggested. These are more spread out and create more water circulation in the tank.
I have seen powerhead attachments available which allows them to create waves as well. But IMO, i rather get a wavemaker since they are generally more "aesthetically appealing" when viewed in the tank.
Good thing about powerheads is, you most likely have one around. Personally I would use powerheads only for surface agitation and wavemaker for internal water circulation, though both can function either way.
