Actually, a Defender in standard trim gets stuck easily in mud. However, once you add locking differentials, it is pretty much unstoppable. As long as one wheel has a semblance of traction, you will move forward. In standard trim, a Disco will be better and easier off road - all the ABS trickery to lock wheels really does work. The Defender is tougher, and if you bash it, it is "patina" rather than "knackered"
I have a 4.6 Defender LWB, and whenever something big or filthy needs moving, it is the right tool for the job. I've had a tonne of horse poop loaded in the back, a couple of diesel engines, tonnes of rubble, 2 hives of bees, you name it. Once the work is done, you pressure wash the interior, and it is as good as new.
The downside? I had to commute in it for a week when the 156 was having cambelt surgery. £300 of petrol in 5 days, and it nearly killed me.