Both down and synthetic fill materials have their advantages when it comes to mummy-style sleeping bags. Down insulation offers excellent warmth-to-weight ratio and exceptional compressibility, making it ideal for backpackers and those needing a highly packable sleeping bag. However, it's important to note that down loses its insulating properties when wet, so it may not be the best choice for wet or humid environments. Synthetic fill, on the other hand, retains insulation even when damp and dries quickly, making it a reliable option for unpredictable weather conditions. It's also often more affordable than down. Consider your specific needs and the expected weather conditions to determine which fill material is the best fit for your mummy sleeping bag.