Electric fan is more efficient and more reliable. It either works or it doesn't. Most of the overheating problems on these cars are caused by the viscous clutch fan that still has some friction but not enough to keep the car cool. There is really no perfect way to gauge whether the clutch is good or not. The fan itself is way too small to have any chance of keeping the M103 under 100c in hot weather or stop and go traffic with AC on. Add a little bit of wear to the clutch and you'll slowly work your way to a new headgasket.
