As a natural fiber like cotton, linen offers excellent breathability. However, linen is evenmorebreathable than cotton, because the fibers themselves are more hollow, meaning air can flow through more easily. Plus, it usually helps that most linen fabrics have a lower thread count — and thus looser weave — than most cotton fabrics.
However, there are lower quality cottons used in sheet making as well. Upland cotton is one of the most common. Because of its lower quality, it is often cheaper than Egyptian or Pima/Supima cotton. Additionally, organic cotton of any variety is generally more expensive than non-organic cottons.