I own the Kelvin Light Jacket in Synthetic fill - I spoke with Sitka and they said the jacket with Down will be a touch warmer. I really love my Kelvin light jacket, it stuffs down super small and is perfect for the September day when a front blows in and I need a bit of insulation. It is always in my pack and has been put to good use many times. I am also intrigued by the Kelvin light down, but feel if I purchased it, I may just be doubling up since the synthetic fill seems to do what i need it to do. The only difference besides that is I believe it is a hoodie, now that will be a bit warmer as well.
I have the Kelvin Down Jacket - and it is like wearing a sleeping bag, super warm and no cold gets through it. I wore it during the spring bear hunting and riding 4 wheelers in cold temps and it was amazing. I could sit out and glass in pretty brutal conditions and it kept me toasty.