I own the Cypress. I purchased it overfilled through Hermit's Hut. My son owns the Bison. Also purchased overfilled through Hermit's Hut. What has been said is spot on. "_The Bison is designed the keep you warm at -40 by using extensive amounts of design with minimal amounts of down_." Whereas, "_The Cypress is designed to keep you warm at -30 by using a minimal amount of design with massive amounts of down_." We are both roughly 6', 160lbs. I fill my Cypress with Nalgenes, electronics and down blankets which improves its efficiency. These are the 2 biggest, baddest bags on the planet, but when it's -30, having one is just part of the equation of a successful night's rest.