The standard Kelvin is heavier throughout than the FL puffy.
Depending on what your preference on a hood is, the FL puffy has a very nice hood with a good 3 way cinch down system. The FL puffy has body mapped insulation, thicker in the over torso for core warmth, thinner in the arms. The hood is insulated over the top of your head and uninsulated over the sides of your ears to not restrict hearing as much. The FL puffy has a nice offset zipper to the side of your chin. An outside chest pocket and two hand pockets. Elastic draw cord waist.
The Kelvin has no hood, like mentioned the insulation thickness is the same throughout in the arms and the torso. The Kelvin is going to be a much warmer jacket overall except minus the hood. Has an inside chest pocket and two outer hand pockets. Elastic draw cord waist.
I believe the Kelvin is at about 23 ounces for a size medium. FL puffy is about 19 ounces for a medium. Both are wind/water resistant with a DWR. Both dry out relatively quickly. I noticed the Kelvin is a little more wind resistant with the thicker insulation, nearly totally wind proof. The FL puffy is pretty much wind resistant over the torso and hood, but the forearms have a more breathable type material and not really wind proof.
I regularly use both depending on the time of year and what I'm doing. I have a love hate feeling towards hoods since I nearly always have a beanie and dont always use a hood.
The FL Puffy was nice in the Brooks Range last year since I could cinch the hood up tight and keep mosquitos out of my face and off my head.
You can do some shopping around online and find deals on Sitka and maybe the occasional sale on FL.