You want to use the program that you fly on most. Especially since promotions only apply to flights operated by the airline that's offering them. If you are flying on US exclusively, then you are better off staying with Dividend Miles. You can always redeem your DM miles for any *A flights.

US DM miles are quite good for using on *A award flights. US does not block availability if award seats are available. US own award seat availability sucks, try getting award flights to Hawaii. But, you can use DM miles on other *A flights.

Unles you're looking to actually fly on other airlines, then you should just stay put with USAir.