To save Gas and Gain HP, if your vehicle has a clutch fan, replace it with an electric fan. Having a relay for the fan that turns it on after a given temperature is most ideal, that way you're not overcooling or prematurely cooling the engine, also it'll allow for quicker warm up time.
I wish I could convert to an electric fan but there are times that even my mechanical fan with the heavy-duty clutch can fall a little behind. Nothing to be worried about at all but it does a bit. When it's hot like that I can't rev over 3k without the belt starting to slip from the fan trying to take too much power.
I would say it may depend on many factors, but I would lean toward the electric fan.
I know Nissan still uses a clutch on the 350 Z. They never went to electric fans.
In defense of their choice:
Clutch fans run between 1500 and 2500 RPM. They also will tend to reduce the pressure build up in front of the car, and keep thins cooler under the hood. This could be especially significant with the twin turbo model, which puts out a heck of a lot of heat. Turbos, intercoolers, radiator, all adding up to a lot of BTU's to cook everything in the engine compartment. The fan clutch is thermostatically controlled, and normal airflow would spin the blades up to a certain RPM if the fan could just freewheel.
Counter their choice:
Electric cooling fans only run when the radiator outlet temperature reaches a level to activate the fan. This is more efficient in my opinion. Electric fans also run when the AC is on to add airflow over the condenser. A clutch fan would also increase fan RPM when the AC was running.
I guess the choice threshold would be depending on what car you were talking about. I would tend to think Nissan did not make their choice becasue either was more expensive.
There is another option. Mercedes used to put an electromagnetic clutch on a flex fan under the hood. When the fan wasn't needed it could just freewheel. I'm sure that leads to better efficiency when you aren't revving the engine all out while the fan is engaged. I realize it's a flex fan but those are still power suckers.
My Cressida I converted to electric fans and had a bypass switch on the fans to manually turn them on or off. If I was on the freeway and under 70 it didn't need the fans, however, as you got above 70 you needed a fan, then above 80 you needed both, and anything past 95 even both of the electric fans weren't enough.
It's stop and go traffic with the ac on that causes my heat buildup. I can be in 4L pulling up the side of a hill and have no problems.