40 oz is the weight of the deluxe model, which is significantly bigger than your air core. You can save something like 8 oz if you go with the std model, and even more if you get the short one and put a pack under your legs.
I guess "worth it" depends on how cold it's gonna be - I've been thankful for the extra insulation I gained over a thermarest on several occasions! I will probably NOT be hauling this thing along backpacking when the lows aren't getting near freezing, though.
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