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.