This is really four questions.....
1. Will you loose the value of your tickets if USair goes belly up?
The answer to this is NO. The same laws that created the ATSB and the 9-11 grants to the airlines protect you if an airline dies, and this has been a custom in the industry even before such laws. Other airlines are required to honor their tickets. You may have to pay a change fee which could be significant with a whole family's worth of tickets.
2. Will your travel plans be disrupted is USair goes belly up?
Almost certainly YES. The danger of this is greatest on routes where USair has the most market share. The chances of being able to rebook a flight to the same destination and time are pretty darn slim if USAir goes belly up anytime within a couple months of your travel date. Airlines run with relatively few empty seats on most routes, most of the time and there will be many people in your situation. Finding seats for a whole family together would be particularily difficult in such times. You will probably have to choose different travel dates. This means you are pretty much screwed if it happens to soon to change the time you get off from work, etc. You could even be stranded for a time if you are unlucky enough to have USair liquidate while you are away.
3. Is there a significant chance that USair will go belly up by this time next year?
YES. In fact many intelligent people on this board believe that there is a better than even chance this will happen. I wouldn't venture to guess myself.
4. Is American less likely to fail?
YES. The likelihood that American will go belly up by the time you mention is very, very slim. A chapter 11 filing may happen by that date but that does not affect passengers. If there is another recession or terrorist attack then all bets are off. If JetBlue flies to your destination I'd pick them. If your destination has a flag carrier you MIGHT also be in better shape since those countries tend to protect their flag carriers - for obvious reasons.
SHORT...American is a pretty safe bet for now, and if the choice is just between USAir and American I'd fly American even if it was a little bit more costly. JetBlue (though not ATA or other lo-co's that go to that region) would be even more desirable for me. I would do this NOT to protect the value of my tickets, but to prevent my travel plans from being disrupted. Getting everyone ready to go somewhere on a particular week or two is a monumental undertaking for most families, and very few can simply change travel dates at a whim if an airline goes belly up.