- Promotions: At the start of 2006, US Airways introduced a new MasterCard issued by Juniper Bank (now part of Barclay’s). For the first year, it gave/gives 1.5 mi per dollar of spend; an initial bonus of several thousand miles; and credited you with 10K flown miles (i.e., tier miles toward status) if you spent 25K in the calendar year. Unlike most miles cards (free only for year one), this card is free for the first 2 years.

- Ratio & flexibility: the Starwood program allows 1:1 exchanges into most partner airlines (UAL being the big exception. Better still, for 20K points you get 25K miles, or 1.25mi/dollar. The annual fee is pretty low ($35, I think), much cheaper than the annual fees for most airline-specific cards. Also, Starwood’s hotel redemption has been a good deal, although they seem to be chipping away at the program lately.

In short, as with financial investments, your optimal strategy is a blended one, IMHO.