Following on from DW's comments above, I'm usually a size large but the size XL Prism 2 fits me well and the length and arm length are just right too. I'm sure I would have found the size L to be too short. For me there is absolutely no point in having an insulating layer that exposes your waist and nethers to the elements, where you'll lose loads of heat. Despite my, ahem, significant beer belly, I can easily layer the Prism 2 underneath the Fitzroy, which is only a size large, for crazy insulation when I get trapped by my arm in the bottom of a crevasse.
FWIW I really like the Prism 2 - I was expecting the 40g/m2 not to be particularly toasty but in fact I'm finding the jacket surprisingly warm. It has a double layer of 40g/m2 in the front covering the long pockets for about 2/3 of the length of the front of the jacket, which probably makes the difference. Surprisingly, I don't think it breathes as well as my ME Fitzroy which is 100g/m2 Primaloft One in the body, with arms tending to get a bit sweaty fairly quickly for some reason. It is totally windproof.
None of which gives you any info whatsoever about the Prism v Prism 2. But hey, it's good to ramble.