I've run both and my experence a mechanical fan just flat works better. Where I'm at getting the car too cool properly is probably one of the biggest challenges when building a car.

What I have found works best (if theres room) is a good clutch fan with as many blades as your can find ( buy a GOOD clutch unit that is also thermostatically controlled). Direct Connection (forerunner to MOPAR Predformance) ran a HP test rears ago and found flex fan ate over 20 HP and the clutch units were down around 9 HP.

Next best fans that I've found are the stock 7 blade Ford flex fans,