I have 40K miles on AA and 57K on US. I'm taking a trip to Buenos Aires with some friends in April, and you can get there on AA with just 40K March - May.

I am wondering whether I should spend the extra 10K and go on USAirways, since they may be worthless soon, but then again, if they do go broke, it may mean the award ticket is useless...

Any advice?

You could book them both and then redeposit the award you don't want to use. Depending on your status the redeposit fee could be nil on US (for CP level). Judging by how few miles you have I am guessing you would have to pay a redeposit fee but it might be worth it as an insurance policy.

My advice is to use the US miles since it's only a difference of 10K.

Break it down mathmatically. Your real decision (barring any extra value you place on getting there with one carrier over another) should be based on the probability of you being able to use the miles on the trip after this May one (as the surviving carrier would need to be able to complete that trip for your miles spent). Therefore if you believe there is a greater than 80% chance your US miles will not be devalued or lost by the time you wish to book a trip subsequent to this one, use the AA miles. Otherwise, the US miles are the better choice. Based on what I am seeing I currently value my AA miles at approximately 1.8 times my US miles for any trip 2-4 months out. You will of course have your own thoughts on the matter, but can devise a similar valuation method. Good luck!

Use the US Air miles on a *A award, nothing else makes sense.