The biggest difference between sleeping bags is often down versus synthetic fill. Both synthetic and down fill can keep you warm in extreme conditions—where they vary is in how much insulation is needed to reach the same temperature rating, and how much space they take up when packed. Less expensive synthetic fills can make affordable sleeping bags that are suitable for car camping. The highest fill power down (800fp and up) is used for ultralight sleeping bags that are typically fairly expensive and used for multi-night backpacking trips or thru-hikes. High quality synthetic fill and lower fill power down tend to meet in the middle, and are suitable for overnights and basecamps.