[Flieger]

This is how I see it:

Torsion bar rate is "constant" for degrees of movement. The torsion bar's force vector changes both magnitude and direction throughout suspension movement. The A-arm length does not change so the torsional force increases linearly. The wheel rate takes into account the effective control arm length for the torsion bar- the perpendicular distance from the ball joint to the torsion bar. This value decreases either side of a horizontal A-arm. This means that the torsion bar effectively gets stiffer (more than linearly) as the A-arm moves by degree. The effective arm length is just to cosine of the angular displacement multiplied by the actual arm length.

Coil-overs act in a near-vertical plane and therefore transmit a relatively constant spring rate force vector direction to the wheel for any A-arm displacement.