In frigid conditions, the bag is going to keep you much warmer.

i'll go with the blanket.with a fire going,a big one,and the blanket as a lean too shelter you could stay warmer and dryer than just being out on the open with any heat from a fire reflecting off the bag.with the blanket you could move around to feed the blaze and sit or lay on some sort of heap of ground cover that would dry out from the fire.the trick to a reflector fire is to make the back logs really high and wide,like hip high at least and as wide as the lean too.

Since traditional mylar space blankets are so fantastically lousy, and the AMK polyethylene Heat Sheets & bivies are so much better, we should drop the mylar items completely from the discussion. The comparisions aren't really worth the bother regarding mylar, whether it is a sheet or a bag.

It's like evaluating "hatchback vs. small sedan?" and including the Ford Pinto in the mix.

i swapped out a foil bag for the Heat Sheet bivy from the good reviews i got on this site.it fits in my PFD and takes up no more room than the foil one and i can get a head net into the sack in comes in.
OK..about those Survival Blankets.i have not tried out the AMK sheet yet and i always take two of the Survival Blankets,with the mesh,on every canoe trip.for my needs they work just fine but i'll put the AMK on the list of things to try out.my local gear shop will have a $10 gift card along so i can buy and try.

The Problem..with Mylar---



this one was exposed to dry heat when i used it under a water bed heater that was warming a vat of crushed grapes while making wine at home.the holes were the part that was not covered with the vat but in open air touching the heater...my fault.



this one was used as a floor mat in my Baker shelter.i had set pots of stew and other boiled items on the blanket with no problems but when i set a fry pan with a bannock right off the stove on it,yup--melted right through.i cut the hole square and use the blanket as a floor and the hole serves as a "cook hole",at least thats what i tell my fishing buddys---

if any one has AMK blankets that have stood up better i would like to hear about and maybe a photo.

I would certainly agree that the Heat Sheets are better, but I am recycling my mylars as footprints (groundsheets) for my small tents. They do quite well in that capacity.