Has anyone done any testing on horsepower differences between these two fans? I have a 7 blade 18" clutch fan with a Jag clutch and an 18" Flex a Lite fan. I've used both with the same cooling results. Just wondering if there is any HP difference between the two?

Do some testing at a track and let us know

Engine Masters did a thing on this and flex fans were generally the worst by a good bit. I think the worst one was a plastic fan at full flaps

Boy you’ve opened up a can of worms now!! The howling will begin soon, stand tight hold your ground!

I've never tested them back/back, but the last car I had with a flex made a fair amount of windmill noise.
I didn't expect that, as I thought the blades would flatten with RPM, but I didn't look into it as it cooled fine and it was a street car.
The clutch could be one that decouples with RPM, or activates with temp: 2 different things.
I would think that a smaller diameter fan would free up some power, but I wouldn't expect much.

x2. Best is obviously an electric fan. Next best is the clutch fan setup. The flex fans are worse for parasitic loss.

i had very good luck using a clutch seven blade fan on a 408. The clutch limits fan rpm, which limits power loss. Flex fans, not so much.

How about a flex fan mounted on a clutch?

Doubt you can find a flex fan that will fit on a clutch.

That kind of defeats the purpose, The flex fan "flattens out" at high rpm's and the clutch limits the rpms of the fan. Electric fan is way to go, No drag on the engine.

Where does the power to run the electric motor come from? From the alternator.
What drives the alternator? A belt from the engine.

Now when the fan is turned off, there's no power required.
How much power does a mechanical fan consume when it's basically freewheeling?

The seven blade moves a ton of air at low speeds, making it efficient because max rpm it will see due to the clutch is tied to its huge capacity to move air.

Dr. Charles is on the money! My street GTX runs 10.80s at 124. It has the Mopar performance 7 blade fan mounted to the Jaguar clutch. I have tried switching to an electric water pump and fan with no change in ET. Now this car weighs 3960 with me at the starting line. So the 10 maybe 15 hp increase is not enough to improve the ET or MPH. I'm sure that on a 3000 lbs car 15 HP will change the time slip numbers.
I've also tried rotating weight at the wheels. I took the exact tires on a set of steel magnum 500s and also on a set of Cenertlines. 38 lbs difference on 4 wheels. Once again THIS car did not show a difference on the time slip.
I know if I add the light weight wheels to the electric water pump and fan then the sum of these would probably gain me a tenth. This is first of all a street car. My race GTX has the lightweight wheels, electric water pump, fans fiberglass parts and so on and it is a RACE car.