In reply to tony:
I have a Terra Nova 4 person and a lightweight Rab 2 man bag.
The standard TN bag material appears far more robust than the Rab bag but is also far more heavy. I have never had to use the Rab bag in anger.
I use both bags but for different purposes and you also need to consider just what you will use it for.
I take the Rab bag when I am climbing/ski touring when it will be used only in an emergency. The heavier TN bag is used when I am guiding in the Alps and snowshoeing in the Pyrenees when it is likely to be used for sitting on/sheltering in adverse weather rather for just an emergency.
Both are superbly made.