Clutched mechanical fan for me. I'm of the opinion that a mechanical drive can transmit more power to a fan than an electrical drive. Additionally, many ExPo vehicles already have a heavier than normal load on their electrical system.
A good clutch will disengage the fan when it isn't needed. It'll still spin, but not with full force or much load from the engine. When idling and cool, I can stop the fan on my Land Cruiser by holding a welding glove to the back of the fan - even spin it backwards. For fording water - slowly entering the water will cool that clutch pretty quickly, causing it to disengage.
When hot, even at idle, it'll darn near suck your shirt onto the grill.
Some mechanical clutches are "tunable", too. In many Toyota clutches (manufactured by AISIN or Eaton), you can replace the silicone oil, responsible for transmitting power from the shaft to the fan, with a thicker viscosity; this causes a "stronger" engagement when hot. I'm currently running approximately 17,000cst oil, versus the 3,000cst oil that is stock in US market FZJ80s. The thermostat valve may also adjustable, which changes the temperature where the clutch starts to engage. My knowledge is limited to what I've seen on Toyotas, but I don't have any reason to believe this wouldn't be true for many other manufacturers.
