Welcome to FlyerTalk.

You pay the fees and follow the rules for the program you redeem your miles in, whether that's United, Continental or US Airways.

You haven't discussed what your travel patterns are. This is rather important to know in order to make recommendations. Do you just casually fly a few times a year for a lark, or are you looking for lots of regular travel for an employer?

Also, since no-one's bothered to correct you on this: you're wrong about redemption levels for lowest-tier domestic coach tickets for US Airways: they are 25,000 just like UA and Continental. (It's also a very poor use of miles compared to premium class redemptions to exotic, far off locales, if that's what you'd like to do with your miles.)

I would not rule buying US miles out of hand. It is possible to buy US Airways miles and get an award ticket in business or first class that would be comparable in cost to coach tickets to locations you would be traveling to (since US Airways is liberal in allowing creative routings). For instance, I laid out perhaps a thousand in cash (after doing various other things to add miles to my US account), and got a first class trip to Hong Kong and Germany out of the deal, going around the world. Other members of FlyerTalk have done similar trips as well. The coach tickets involved in my trip next year would in all likelihood cost something over $1000 by themselves, and would be far less comfortable.

That being said, this all goes back to where you expect to travel and what you'd like to get out of it. If all you want to do is go to Las Vegas, New York or Disney vacation parks for free on a vacation, for instance, you want a different approach than needing to be able to fly to Chicago on a moment's notice for your employer, and wanting to sit in the first class cabin on those last-minute trips without paying for first class tickets, which is different from saving up miles to fly around the world for 3 months.