First of all I would add that I feel that this discussion must be expanded to the Suspension set up. Logic is that springs work in conjuction with suspension geometery and other hardware (axles, locating arms etc) to give it the feel and strength you are talking about.
My suspension of choice is rigid/ solid axles with coil springs. To elaborate further I would like to add that variable range/ tension coil springs with variable damping shocks work best. The usual setup we see in bikes and cars do have variable rated springs with only one setting for the shocks. In high performance setups not only the springs progressively get stiffer as they are compressed but also the shocks have 3 or 4 stage valving (soft-medium to hard) that add resistance as they too are compressed.
Second part of discussion deals with the amount of axles articulation engineered into the suspension geometry. The more the articulation the more a wheel is able to travel up and down and follow the terrian while keeping the vehicle straight. In good 4x4 with rigid axles and coil springs front and back axles are able to reach extreme angles while the vehicle is fairly straight. This ensures maximum traction, control and safety during extreme angles, especially over rocks and boulders.
In terms of comfort and reilability coil springs win...any time. Leaf springs are great for exteremly heavy loads and are very durable durable under extreme loads, however, as COIL springs do not have multiple components they are still more reliable.
Defender setup rigid axles with coil springs is a good setup in my opinion.
The above is only my opinion. I am sure experts out there can do a much better job in explaining springs and suspensions.