If you redeem 20,000 points at the same time, the Starwood card gives you a 25% bonus and you get 25,000 airline miles. This may work out to more of a bonus than double miles on things like gasoline and supermarket purchases, though if you buy a lot of Delta tickets the Delta card may be worth the $85. Figure out how much you spend with double-miles partners in a year, how many extra miles that works out to, and what those miles are worth at your favorite value per mile.