POST *EDITED* TO REMOVE ***LIES*** about Toyota Hybrids versus dusty dirt roads.
Denny set me straight on the next page: they say that you shouldn't use the HiHy as an OFF-ROAD vehicle, but they don't say why, and they don't ban dirt roads.
The rest is OK, and I leave unchanged.
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The 4Runner is built on a FRAME, like a truck, while the Highlander is built as a unibody, like a car. That's a key issue behind Toyota's much lower Tow Rating on the Highlander. Engine-wise, torque is king and the HiHy utterly kills even the V8, because an electric motor provides maximum torque at 0 RPMs (while gas motors have to "rev up" to get into the proper power range.)
But the 4Runner V8 drivetrain is BRUTISHLY strong: The tranny cooler is factory installed, and Toyota is so confident about the temperature management that they don't even have a dipstick or filler to mess with/replace the Tranny fluid-- they intend it as a lifetime, never-replace-the-fluid system. (In spite of Toyota's design, I'll probably do a fluid replacement at 50 or 60K any way.
If you're gonna consider the 4R, I strongly recommend upgrading to a model with the X-REAS passive hydraulic suspension feature: it does an amazing job of smoothing out the ride, and keeping all 4 tires on the ground, when you're going over bumps. That includes both in-town and off-road-- without the X-REAS, you'll always be knowing that you're in an off-road vehicle while you're driving around town.
If it wasn't for the overhang (front and back), this would be the PERFECT off-road vehicle. The overhang does limit approach and departure angles, also the tires not being set real wide (track) limits the tilt which you can handle in comparison with, say, an H3. But the H3 5-cylinder engine is grossly inadequate, I've swapped vehicles on some BLM land with a friend, and he turned sickly green with envy and shame (at having wasted his money on the Hummer) as he quickly realized that the 4R's power made it a better off-roading vehicle than his, even though the H3 is at it's best doing off-road-- it's horribly nasty to drive on the street.
The V8 4R has a towing capacity of 7K, that's serious overkill. In the real world, the short wheelbase is the limiting factor. The V6 has a less capable hitch receiver, it's not bolted to the frame in as many places and it's less "beefy". But at a 5K rating, and Toyota is IMO being very conservative, that's overkill too.
The Highlander is certainly adequate-- and I'll guess that the electrical motors DON'T loose power at altitude, although the gas system will be somewhat affected.
I chose the 4R, because we do mountains. Heck, the thing hardly ever went into 3rd gear along 395, and with 4 valves, the engine is happy to rev pretty high. It didn't have to. Towing a little TM behind the 4R was a non-issue, even through Washoe Valley when the "high winds, trailers not recommended" lights were on. There's lots of semi rollover accidents in that stretch of road, but I was barely even aware that the LOW, STABLE, TM was back there. (I do have WDH and a great brake controller.)
Both are also listed among the top 10 of *ALL* vehicles in the IIHS vehicle death rates publication-- the 4Runner is in second place, just behind the biggest Mercedes Sedan. I'd take the real world statistics over the front and side crash tests. It's too bad that they don't have an update for newer model years, but these are incredibly safe vehicles. The 4R killed a few people in roll-over accidents, while the Highlander/LS300 killed more people in multi-vehicle crashes (weighing less, and being a unibody, the passenger compartment is less strong).
Either of these is more than 5x less likely to leave you dead in a crash than a typical Sedan, Truck, or SUV. Outstanding!
My Summary: The 4Runner has far more towing capacity, but you can do pretty steep grades in the Highlander too. It's really a question of style-- if you can stand the "SUV Look", the 4R comes with more capability, and the suspension is as smooth as a car (a GOOD car.) But it does have that big step up, and you're riding high off the road. If you want less of a "truck" riding stance, get the Highlander.