The clutch fan free spins when the engine is warming up, and only drives when things start getting hot (when working properly) therefore saving the engine from wasting horsepower (and fuel) driving the fan when it doesn't need it. When your engine is idling, you should be able to stop the fan while it is free wheeling, but I wouldn't recomend doing so;p
In my opinion a cluch fan is far better than a fixed fan (when working properly) I'm not sure, but I think the mounting flange on the water pump is different between the two types. Maybe someone else will know for sure.
I personally would not bother putting on a clutch fan.
Even clutch fans rob HP. If you want to free up some horsepower go to electric fan set up. A buddy of mine did that on a 4runner. Worked pretty nice. His 4runner has a 22re engine, and he said he felt the power change.
If look on here many guys also pull the clutch fan and istall a fix one too. They say it cools better and is more reliable. As the clutch fans need rebuilds once and awhile.
Anything attached to a moving part leeches some power - like an alternator, a/c compressor, fan etc., but I think it is nominal for a clutch fan. I personally like clutch fans, i just bought a new clutch and fan and tossed the fixed fan on my 1hz - I cross water so dont want a fixed fan, I also have never had a problem with clutch fans. The new fan is much bigger than the fixed fan that was on i originally - lots more wind off it.
Keep your engine tuned, rad clean, thermostat functional and you can run either type with no problems.