What's the difference between a touring sock and a lightweight spring ski sock?

While both are thin and breathable, touring socks include features that spring socks typically lack: extensive ventilation zones designed for high-output activity, moderate compression for fatigue reduction, reinforced zones for the walking motion of touring, and antimicrobial treatment for multi-day use. A spring sock is essentially a thin resort sock; a touring sock is a purpose-built garment for the ascent-descent cycle. The price difference reflects the additional engineering.