Let's just compare Subaru Forester and Nissan X Trail
The problem is not all 4wds are created equal in fact far from it. In the most basic of four wheel drive there is a centre differential that transfers torque from front to back (i think). In the simplest system it doesnt transfer enough torque to the rear wheels and you might as well not call it a 4wd. sometimes the centre differential gets too hot and it shuts down altogether, and you're back to 2wd.
Between each axle front or back, in the simplest system if the right front wheel spins because it has no traction, progressively more torque is sent there depriving torque to the front left (the path of least resistance??) so you have no drive at all!
So how does the Nissan and the Subaru compare? (I'm comparing arguably the top two but i want to know which is better ...)
Nissan X Trail 4x4i is front wheel drive and sometimes sends power to the rear when needed AND when it thinks its needed. It can transfer between 100/0 % up to max 50/50 %. So if you really need the torque at the back - bad luck. And it can only do 50/50 up to 40km/h, so no good on the open road. What if there is right -left imbalance? If the right front is slipping the computer brakes the right front, increasing the resistance to torque to all the torque is diverted to the left front where there is friction. same if happens in the rear. Problem is it doesn't brake the wheel all the time, only in pulses so it may or may not get you out of trouble.
Subaru: i have no idea .... but people (AUS X Trail forums) rave about how it is superior ...
For one it has full time four wheel drive. In fact the centre differential doesn't actually transfer torque from front to back. It is connected directly to the engine so it distributes torque between front and back (subtle but important difference perhaps?).
But can it go 90/10 to 10/90? and what happens in left - right imbalance? Thanks if you know