Pay the extra and get a sleeping bag that will be waterproof and warm over a wider range.
Vango venom/viper range and the Snugpak range are all great if a bit pricey. Warm, pack small and are light. Just google them and you will see loads of information on them.
If you want to pay less and are willing to carry more weight and size then look for the latest ex-military British or American army bag. Very warm, waterproof (gortex ones). The British ones have a central zipper which is handier than it may sound. Go for the large if you go for the British military one as the medium only comes up to your chin. Google ex-military uk and you will see all you need to know.
Personally when I went on deployment I would use my own bag (Vango) and I was always warm. Only used the military one when I was in the arctic.
Hope this helps
