In reply to wbo2:
This one I can roughly compare, though with different generations of jacket. My beloved and well-patched Patagonia nanopuff is c.2014 so uses primaloft ONE (predecessor of gold) and it may well be that the upgraded model is more durable, though it still uses similar insulation and design. It was my go-to everything insulation for 4 years. I will also add the Rab Xenon X as my other lightweight synthetic.
Disclaimer: I am female, tend to run cold and am really bad in wind. Therefore apply that overlay to comments below!
Personally (and apparently inexplicably from what I read elsewhere on the internet) I have found the nuclei (65g Coreloft) significantly warmer than either my nanopuff (60g primaloft ONE) or my Rab Xenon X c.2017 (60g primaloft Gold). Both are similar weight (no more than 12g heavier) hooded jackets in my size.
I also think the face fabric and construction has a big impact in this department - the brick stitching on the nanopuffalways made that less windproof and provided weak points for insulation for me. I have grown to appreciate the benefits of a continuous panel for keeping me warm, though obviously harder to patch and repair. Otherwise the nanopuff retained warmth when when soaking wet and packed small and light. I have never really got on with the Xenon X in the outdoors. It is better keeping wind out than the nanopuff (which I attribute to being a continuous layer of fabric) but I just can't seem to heat it from the inside. It packs small and the outer repels water well but when wetting out gets quite sticky against any skin.
The inflatability/loft of the nuclei is hard to describe (I tried in the review!) but it is also complemented by the outer's windproofness and this means that you get the windsock (see review) qualities when forcing air through which makes it loft more. Even sticking an arm in and displacing air means it gets all fluffed up. (In terms of insulation feel I'd call the nuclei 'fluffy', the xenon 'ethereal' and the nanopuff 'denser'). The flip side of this is that the nuclei is definitely the least breathable of the three I'm talking about though still better in this department compared to tech of 3-4 years ago.
It's worth saying my husband has both nanopuff (smock, c.2011) and Xenon X (c.2017). He runs warm and finds them both warm. In terms of use he usually uses the nanopuff as a climbing in jacket, and the Xenon as a belay jacket.
Use-wise I would (and have) climbed happily in both, but tend to use both as a chuck on layer. The stitched panels in the nanopuff (and shorter arms) mean the arms are nicer to climb in as there is less material in the way.
The more tube-like stuff sack for the nuclei was fine racked and being light didn't throw my weight off, but the more flat packety pocket stuff of the nanopuff is my preference for racking, possibly because I am used to it! Both are a fight to stuff as they pack tight. The xenon x is an easy stuff into a packety pocket shape and one feels it could have been a little denser to rack better.
For me the nanopuff outer shell has always been a bit snaggy when worn - so far the Arc'teryx outer hasn't. My nanopuff outer also wore through over time (ice climbing, grit rash etc) and I haven't managed to give the nuclei that kind of traffic yet. I'll update the review if I manage to smash it! The Xenon X outer is tougher than it looks, but as I don't feel as warm in it as either of the other two, I don't wear it as much for activities and therefore it hasn't been tested properly in this context.
Again, that's just me, but hope it helps...