Good point Hilltool - lower center of gravity combined with less side wall means less impact from cross-wind (actual wind or pressure cause by passing vehicles).
From a fuel-efficieny standpoint, Treeclimber, it really depends on the type of driving. With short-trip or slow/stop and go (or hill climbing/elevation changes), total mass will be the biggest influence, as the tow vehicle must accelerate that mass, perhaps repeatedly.
Flat-road cruise will be more impacted by drag, since you won't be accelerating the vehicle as much - accelerating forces will be to overcome drag.
Try towing an empty box trailer, and then a similar weight trailer that's smaller (shorter/narrower) than your truck. Under about 45 mph they'll feel similar, but as your speed increases the box trailer will be more of a *drag* (haha...I couldn't resist that one!).
So for the guy that only goes to the park 20 miles away one weekend a month, a regular trailer is fine. For the more road-trekky types, reducing drag with a Hi-Lo can make a big difference.
I live in Colorado - trailer mass will be significant. I try to keep trailers under 3000lbs. Even then I'm not accelerating up mountain roads - I'll be the guy in the right lane with the trucks on I70 (I have a 1/4 ton truck with 250 hp/250lbs torque).
Just my .02, based on years of doing dumb stuff, (and a bit of college physics).