I have enough miles in both US Air and in United to get a round trip ticket to CA this fall. Both share flights, so it would be the same flights regardless of which miles account I used. I would normally purchase the ticket outright instead of using miles, but all of the outright purchases have connections and bad flight times, the miles tickets have better itinerary for me. So my question is, is there a reason why I should keep my miles in one vs the other? Use the USAIR or United miles or does it not matter in the slightest? The only reason I am inclined to use United is because my husband tends to travel on USAir the most for work, so the chances of him getting more mileage to add to that account are better (we will have about 20k left after the purchase).. Where as United, we could use up all of our miles and be down to zero and that would be fine.

In your scenario I'd use the UA miles for the trip to CA. If you're not adding a lot of miles on a regular basis, having two accounts just means more things to manage and a diluted value for each program which makes future redemptions harder. Theoretically your husband could credit his US travel to UA mileage program (as long as it is his UA MP account and not yours) so that is another option but if you ever wanted to do a third-party redemption such as redeeming US miles for an Air Canada flight, the US third-party awards are a little easier to obtain (you will have to do some homework on either program to find the partner award availability) and the US miles tend to go a little further towards partner awards. In other words, it takes fewer US miles to book a partner award than UA miles but both programs will make you do a lot of the research on your own.

Considerations include

• Fees: US and UA have different award redemption fees.
• Online booking: It's easier to book award travel on the "home" carrier whose miles you are redeeming, as this can generally be done online. Partner awards typically require a phone call.
• General utility: Overall, US miles have more value even though both airlines are Star Alliance members. This is because UA routinely blocks partner award availability, unlike US.
• Award chart differences: If you're looking for a particular international award down the road, one carrier may charge fewer miles for your desired destination, in which case it would be preferable to accrue miles in that program.