Why is the Slant so much cheaper than the Quadrant? In the description it mentions the urethane shell. How will that affect the condition of the ski boot (i.e. durability, insulation, flexibility)?

Well for starters, the slant is slated two spots down in the BD Efficiency lineup. The Quadrant is their top end boot, followed by the Prime, and then the Slant. It has a softer flex, only 110 versus the Quadrant's 120 flex. Also, the Quadrant has a BOA liner versus just a regular lace up liner in the Slant, which I actually think is plus for the Slant, as BOA's are a pain to operate, and you're in bad shape if it breaks on you. Finally the Slant has a urethane liner versus the Quadrant's pebax liner. Both are very strong durable plastics, the main difference being that pebax's performance is not affected by temperature as much as urethane is. The colder it gets, the stiffer urethane gets, and the warmer it gets, the less stiff urethane gets. Pebax's stiffness stay pretty consistent regardless of temperature. Finally the slant only has 3 buckles, while the quadrant has 4. If you really like these boots, I would go with the Prime, it's a slightly softer, 3 buckle version of the quadrant. I think it's the best of both worlds. Great deal, similar performance to the quadrant.