You have 2 standard choices when it comes to insulation; synthetic or down. Down filled bags bag will be lighter and easy to compress smaller for space economy, but more susceptible to wet weather than a synthetic filled bag. Down loses most of its insulation capacity when wet, synthetics don’t. Synthetic bags are usually cheaper but do not last as long. But you can pay a few extra bucks and get the best of both worlds buy purchasing a bag with the new waterproof treated down.