Based on an average use of 5 minutes, the cost of using a hair dryer is less than 1 cent per use. Both types have a narrower stream than conventional shower heads, so more water streams directly onto your body, making the shower more efficient. Aerating models add air into the water stream.
Some models offer a shut-off valve that keeps the water hot while the flow is stopped.
This allows you to turn the water off while you're applying soap.