Cotton base layers freeze you and reek. Merino wool keeps you warm, dry, and isn't disgusting after two days.
Why Not Just Buy the Cheap One?
Merino wool actively wicks sweat and regulates temperature; cotton absorbs moisture and stays cold, then holds bacteria that create odor after one wear. Smartwool merino stays fresh for three days of hard use because the fiber naturally resists bacterial growth and dries 50% faster than cotton.
The Buy-Once Math
Cotton long underwear needs washing after every single wear, driving laundry costs and degradation. Smartwool lasts three wears between washes at the same price as two cotton sets ($60 vs. $40-60). Over a winter season, you're doing half the laundry and replacing base layers every three years instead of annually. You'll buy six pairs of cotton before two pairs of Smartwool reaches end-of-life.