In reply to Scarab: I dug out the email from Neil at RAB when I asked him exactly that question. He wrote:
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The Photon / Generator Alpine contrast is as follows:
Generator Alpine: Pertex Endurance (coated) shell, 100g Primaloft 1 (silicon treated water resist primaloft) insulation, helmet compatible hood.
Photon Jacket: Petex Microlight shell (ie not coated), 133g Primaloft Sport insulation, standard hood.
Thus the Gen Alp is more of a lightweight belay jacket with water resist outer shell and siliconised primaloft, whereas the Photon is slightly warmer, not as water resistant, but more breathable if you were actually moving around in it.
The Photon is also better priced at £110 as opposed to the Gen Alp at £140. Basically I would use the Gen Alp on the Ben and the Photon knocking around on Grit.
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I would add that I found the Photon hood goes very well over a helmet and I'm perfectly happy using it as a belay jacket.