Is pure merino or a wool blend better for heavyweight base layers?

Wool blends (typically 60–80% merino with polyester or nylon) are generally the better choice for heavyweight base layers. The thick merino fabric is more prone to pilling, seam stress, and sagging than lighter weights, and the synthetic component in blends addresses these issues while preserving most of merino's warmth and odor resistance. Pure merino heavyweight has a slightly softer hand feel and marginally better odor resistance, but the durability trade-off is significant at this thickness.