If I had another choice I would use the other. I have the LT and like it a lot but it’s more of an active insulation piece and less of a stationary puffy. It doesn’t stop much wind either. I tried using mine early September in Montana and was cold because it was windy.
I wore mine in CO 1st rifle. Over a base and mid layer it works well when slowly moving, or hiking when temps were just above zero. But I had a Kifaru LPP to go over when glassing. Short answer, no, not suitable as only insulation layer.
I run mine all year and is one of my favorite pieces. I’m a Washington guy but late archery elk last year November something I used it a lot and I’d put us in the twenties at times. I was wearing a polar fleece layer like a r1 when sitting and glassing with it though. Might be a bit cold by itself