Warmth in tents vs. tarps & shelters

I'd like to hear your thoughts, comments and observations on the use of a tent vs. a tarp/ shelter in winter.

Somewhere I read that a double wall tent will be warmer in the winter, as the inner is trapping some of the heat. It also obviously protects you better from wind. On the negative side, they're more heavy, and that heat can result in condensation. Cooking inside is also not advisable.

A single wall shelter or a tarp have the advantage that they're a lot lighter, should be easier to keep condensation free, and given the right design they should shed snow as well as a tent. They don't have a floor, which might or might not be a problem.

I'm considering using a tarp or shelter for my winter tours. I'm considering a MLD DuoMid because it seems to be well made for the conditions, or a similar kind of shelter/ tarp. I usually camp in forests, so I can camp beneath trees and wind places which are less windy. Temperatures are between -20°C and -30°C, and snow can be from a few cm to 30 cm or more. I currently own the Scarp 1 with the crossing poles, but am wondering if I am able to get away with something lighter. A silnylon DuoMid + my bivy should be around 640 g, that's a kilo lighter than the Scarp 1. Anyhow.

I'd like to know if there's any truth in the argument that tents are warmer in winter, or if its just one of the many outdoor myths. I also would like to hear your comments and experiences with tarps and shelters in winter, and what you think about my idea for my conditions. Also, feel free to hi-jack the thread if you have related/ similar questions =)