[mikegreen]

There is no way a car with only the ability to power the rear wheels can compare to an AWD drivetrain.

Even with the ability to pulse/modulate the brakes and keep the car from spinning, it will only prevent sliding off a cliff by taking you around that corner at 8mph (notional absurdly low speed here). That is because it will only be able to use the braking + lateral portion of the front tires available grip. If there is no need for braking, just lateral grip which isn't going to be much on a poor snowy road, anyhow. The rear tires have to work doubly as hard in this scenario as well - they have to maintain lateral grip while accelerating (or simply putting power down enough to maintain speed).

A true AWD (or 4x4) would be able to use the acceleration and lateral grip, meaning, the tire can be accelerating and turning. This takes the load off the rear wheels for putting down engine power. Thus, to move at the same speed, the rear tires only have to use much less of their available grip for acceleration and can concentrate on lateral grip (not fishtailing/oversteer situation as above RWD would). An intelligent AWD system will transfer power to whatever wheel has the most grip as well.