Personally... I hate flex fans. They are only rpm controlled. This means they have no idea what the engine is doin workwise.

Examples:

Low gear hill climb: High rpm, hard work, lots of heat, flat fan = no air.

Long gradual freeway downhill: Low rpm, no heat, maximum fan = why?

Both electric and clutch fans deal with these types of situations because they are temperature controlled as fans should be.

Electric fans have a few drawbacks. They use a hell of a lot of electricity! 20 to 30 amps is not unusual. This is not a minor increase in a vehicles electrical budget. Often it requires eventual electrical system modifications. Often you soon discover any electrical system shortcomings. They often mount with strange radiator modifications, hanging the fans on things plunging into the very radiator cores. This creeps me out in the age of $700 radiators. On the upside they cool a lot at idle if needed, (helps the A/C). They can cut all fan related energy consumption at freeway speeds and in cold weather.

Clutches: They can allow a very large fan load if desired but shut down when not used. As mentioned they can fail in nasty subtle ways. They are a heavy item to rotate quickly.

If you want maximum reliability a solid fan is it. Big loud!

Next would be a clutch which drops the noise level and increases the gas mileage.

Electric when engine mods or other things preclude other methods.

Fiberglass... Sim pel, cheep.