You get what you pay for. I had used military surplus bags for ever. They were warm enough. Then I went to light cheap bags. I froze my *** off. Since I found myself getting further from the truck and consequently colder, I purchased a Mountain Hardware Phantom 15. Best bag I ever had. LIGHT and WARM. Seems a lot off people overlook this part off their gear system.
Quick advice
Down: Light, Warm, doesn't do well wet.
Poly fill: Slightly heavier, stays warm when wet.
If you plan on sleeping in a tent an not get your bag soaked go with the down.
Down fill ratings from 400-600 are ok. 600-700 are good 750+ is best.