Rowan and I have good waterproof breathable rain jackets with long pit zips ... so we can climb with the zips open, and then close everything up for the descent.
I also tend to go with polypro rather than merino wool. But definitely not cotton!
My jacket ... Groundeffect's She Shell
http://www.groundeffect.co.nz/produc...il-SHE-RAI.htm
Rowan's jacket ... Groundeffect's Storm Trooper
http://www.groundeffect.co.nz/produc...il-STO-RAI.htm
They are waterproof, and the underarm zips make them breathable.