I'm no pro but I recon softer inserts are like less air in your tyres and harder inserts are like more air in your tyres. With softer inserts the tyre contact patch will be bigger and the tyre wall will deform more. You'll also have slightly lower ride height. Harder inserts will have better bump absorbsion but you'll get more tyre rebound and more deflection. You also need to think about the size of the air breather hole in the rim, combined with the insert hardness and the tyre choice all in relation to the surface traction.
Schumacher now do a green medium insert which I have bought but not truid yet, it sure feels nice
