How would the nano puff or nano air compare to a thermoball? I use it to layer under a TNF apex storm shell or when not wearing the thermoball use the Apex universal soft shell liner that's included as the triclimate jacket.
My thermoball hoodie is warmer than my nano-air jacket (no hood), even with the thermoball hood down. The nano-air fabric has a nicer tactile feel and is more comfortable. Nano-air is much more breathable than the thermoball jacket. I can and do run in the nano-air. I hike in the thermoball and more because I want the hood instead of a hat, otherwise the nano-air would be more desirable to a point, as I said thermoball is warmer.
However I often wear the thermoball hoodie around town and get lots of use out of it. Like it so much that I bought my Mother one last year! I also like wearing it to concerts because I can stuff it into its pocket, clip it to a belt loop and forget about it till it's time to go home.
Thermoball is often kept in the car to wear to and from the ski hill. It also makes a great pillow on flights and wherever it is I'm staying on ski trips.
I have yet to use the nano-air under a shell skiing. Have used the thermoball, I wasn't so active that sweat became an issue and it was ok but not warm enough for me - many reasons for that, one being i don't like bulky layers and was asking too much given the conditions.
As to fit, I'm very fussy about not getting armpit wedgies or strangulated by a tight collar. I'm a small in both but thermoball is roomier. By comparison, I'm a medium in arc'teryx and like their slim athletic cut stuff, in pants too, love my arc'teryx cessair pant.