I love the Kelvin Lite but don't have any experience with the Firstlite. I will say that the Kelvin Lite is great for late Sept/Oct for evenings and glassing but wouldn't be my choice for real late season unless you had decent layers. It's a great balance of being lightweight/warmth but I might want something warmer if I was glassing in late Nov/Dec etc.
I own both, with just the jacket and nothing else the FL seems a bit warmer. However, it does not layer as well for me as the Sitka. In December I was wearing a Sitka merino base, the Sitka kelvin, Sitka Jetstream vest and 90% soft shell over top and stayed warm and was still able to shoot my bow. I run one size smaller in FL than I do Sitka
Since the puffy goes under the shell why not look at a patagonia, sierra designs, eddie bauer, etc... I can pretty much guarantee you that you will get a better jacket for your money with an outdoors brand.
i have both but my sitka lite is the non hooded version. i think they are pretty close in warmth but I believe the FL is a little more wind resistant. I love the FL by itself but its easier to layer with the Kelvin lite.