Hill Descent Mode (or whatever your vehicle manufacturer calls it) is an electronic system designed to emulate low-range four-wheel-drive. It isn't the same, although it's pretty good in most on-road situations and many off-road, too. Low-range four-wheel-drive uses gear ratios to adjust how much power goes to the wheels in relation to the engine speed. As a result, as your engine revs up when you go downhill, the wheels don't turn much quicker, controlling your descent. In contrast, Hill Descent Mode uses a combination of sensors and existing parts, including ABS and traction control, to slow your speed as you drive down a hill. Essentially, it pumps the brakes without you necessarily realizing it. In short, a car with Hill Descent Mode is better suited to the road, while you'll probably need a 4WD low-range transmission setting for any intense off-roading.