Hard-sided carriers are made of durable materials like plastic, offering sturdy protection and ease of cleaning. They're generally more secure for travel and in case of an accident, providing better protection for your cat. Soft-sided carriers are typically made from fabrics like nylon, making them lighter and more flexible for storage and carrying. While soft carriers can be more attractive and less intimidating for your cat, they may not offer as much protection during travel and can sag, potentially making your cat feel less secure.