Will Rietveld's recent excellent review of the Terra Nova Laser Ultra 1 included a chart of inside/outside temps over time. I find this info useful. Here's why.
Other things being equal I'll choose the warmer shelter (tent, bivy, tarp, whatever) because a warmer shelter will allow me to carry a lighter sleeping bag.
For example, my current 1 person double wall tent is routinely 10-15 degrees warmer than the outside temps while I'm sleeping. I don't have good data on other shelters but, for discussion sake, let's say that this is about 10 degrees warmer than, say, an open tarp set up.
Looking at various sleeping bags at Western Mountaineering it looks like there is nearly a half pound difference between bags with 10 degree rating differences (e.g. a 30 degree rated bag compared to a 20 degree rated bag).
So in some situations I may be tent-weight wise and total-weight foolish if I save ounces on my shelter only to find that to keep warm I must add more ounces to my sleeping bag or clothing.
