I think that unless you do a lot of flying on one single airline and/or live in one of their hub cities then you may be better with a cash back card that gives you something that you can use at any time in any amount for any purpose. Perhaps an exception to this would be if you're just trying to get enough miles to get a ticket with your 10k and thus not let them go to waste.

Also, make sure that places you normally use a card accept AMEX as many places only accept Visa/MC and do not accept AMEX. All depends on your spending habits.