I usually wear Marmot fleece jacket which...
I usually wear Marmot fleece jacket which is polartec 200wt under Stingray jacket with just a thin base layer. I am usually comfortable or even a little too hot except in windy conditions. How is the warmth in Atom LT compared to the Marmot fleece jacket and is it much better in windy conditions in terms of wind proof? How about breathability? Should I go for Atom SV?
The Atom LT Hoody is about equivalent in warmth to a 200 wt. fleece, but with much less weight and bulk. It does have a wind/weather resistant face fabric, and a smooth fleece under the arms, which give it a much broader usability than any regular fleece. The fleece underarms will let a bit of wind come through, but helps keep the breathability good. A couple other things I like about this (I've basically retired my fleece for layering) smooth face fabric slides under shells, especially Stingray with it's fuzzy inside, and it's super packable if you don't need the insulation; about 1/3 the size of a 200wt fleece when packed. The Atom SV is more of a wear alone insulator in warmth and fit.