The best sleeping bag liner material depends on where and how you travel. Cotton liners often feel soft and familiar, which many people like for guest rooms, cabins, and everyday comfort. Synthetic options usually dry faster and pack a bit smaller, which can be useful for camping or frequent travel.
Neither material is automatically better for everyone. The right choice comes down to whether you value softness, drying speed, packability, or easier care on the road. Thinking about your real trip conditions will usually lead to a better purchase than focusing only on specifications.