Merino wool and polypropylene are both popular materials for men's thermal wear. Merino wool is a soft, natural fiber that is known for its insulating properties and ability to regulate body temperature. Polypropylene, on the other hand, is a synthetic material that is lightweight, moisture-wicking, and quick-drying.
Merino wool is a popular choice for those who want a natural, sustainable material that is soft and comfortable against the skin due to it's breathability. It is also naturally odour-resistant, making it a good choice for extended wear. Merino wool thermals can be more expensive than polypropylene thermals, but they are often considered to be more durable and longer-lasting.
Polypropylene thermals are a good choice for those who need a lightweight, sweat-wicking material that can handle intense physical activity. Polypropylene is a synthetic material that is made to be durable and moisture-wicking, making it a good choice for outdoor activities like hiking, skiing and paddling. Polypropylene thermals are often less expensive than merino wool thermals, but they may not be as warm or as comfortable to wear for extended periods of time.
Ultimately, the choice between merino wool and polypropylene thermals will depend on your personal preferences and needs.