Remember that you're probably comparing 20 year old used boots to brand new material- of course the old boots will feel softer...They're fatigued and breaking down.

GKN is OEM. The same thing BMW stamps their logo on. They are not hard plastic but not rubber either. Rubber would split in no time. New OEM shafts have pretty stiff boots. The really soft rubbery boots come on the cheap Chinese shafts.

Use GKN and be happy.