I've also got the Rohan Spark Vest, which I couldn't help buying once I'd picked one up in the shop. it is so stupidly light. The Montane Prism is similarly spec'ed (on paper) but is 40% heavier for the addition of pockets, that's a personal choice to make.
I'd highly recommend the Spark, although I used it mostly this spring in combination with a (long sleeved) base layer and a (long sleeved) microfleece to provide flexible levels of warmth. I've never understood the attractions of down vests or 100gsm primaloft vests like the Generator, far too warm in the body and/or cold in the arms for me.
Edited: 03/07/2012 at 11:12