Quick question for the experts here. Which way is the best?

Mrs. cfischer and I need a BOS-YYZ flight which is horribly expensive (USD 300 for the one-way!) and is a 'light fare'. We'd like to check one bag each which will be 50 lbs or under.

We each have UA*G, but I also have AC*S(25k). Does it make any difference whether I add both of our UA*G to the reservation or should I rather add my AC*S? All I primarily care about is to get the free checked bag, but as a bonus - would we get a seat selection benefit if I use my AC*S instead?