
I have the prism, although I'd struggle to make a definite comparison with a fleece, as they're different animals. If I was pushed, I'd say it's no warmer than a 200 fleece. I run hot and find mine too hot to walk in except on very cold days (remember them?). I don't think it's as warm as the generator (less insulation?) but is more versatile as it is a jacket (not a smock) and has a hood, which I find really useful (I don't get on with hats). It packs fairly small, is light and can be put in the bottom of your sack in the cooler months and forgotten about. It's great at rest stops and even shrugs off a bit of rain. It's one of my favourite bits of kit and I chose it over the generator for the reasons above.
Hope this helps!