i have a fidelity amex card that i use for almost everything. i saw an ad for delta skymiles that says "always double miles, plus 15k bonus miles."

if i dont care which airline i get the miles for (i'm quite happy with delta), and i charge $15,000/yr easy, then isnt two better than one? (excluding conversations about the yearly fee (comparable) and credit/debit)

if im going ot use one card all the time, is there a better deal out there than DOUBLE miles?