In general we consider a powerhead to have a concentrated flow via a nozzle like the standard MaxiJets that have been around for decades. They can often be used as a small return pump and will pump against a certain amount of head. Most go back to the days when we had undergravel filter plates and lift tubes in our tanks and sat on top of the lift tubes to create flow down through the gravel or crushed coral substrate.
What you are referring to as a circulation fan is usually a more open impeller or propeller design so it has a wide dispersed flow and moves much more water. It is not concentrated in a jet stream and is easier on most corals. Tunze was probably the first major developer of circulation pumps but many newer styles and manufacturers are in business today.
They both have their uses but for turnover in the display or circulation I much prefer the newer style circulation pumps. Most draw far less power, move many times the amount of water, create less heat and are smaller physically than a comparable powerhead. Another advantage is some like the MP series and WP series are DC powered so you can vary speeds and flows versus AC powered pumps that are either on or off and constant speed.
