This situation hasn't come up much for my wife and me. But if it did, our decision would be situational...

Since I'm the travel coordinator for the two of us and can generally plan far in advance when award and upgrade availability is ok, I always make sure that when we're traveling together cross-country we're upgraded for domestic (using AA miles and co-pay) at time of ticketing or obtain J or F award seats together for international.

There may yet be scenarios (such as award availability constraints) in which one might have to fly in one cabin and one in another for international vacations. In that case - such as an upcoming HKG trip - I'd definitely make sure she gets F and I get J (unless of course an F seat opens up for me also ).

However, one time when it was a choice between F and Y for cross-country domestic travel, she insisted I take F because I'm 6'2" and have a bad back and she's 5'4" and when traveling alone domestically or internationally is thoroughly indifferent to seat assignment or even being squeezed on a domestic discount carrier.

If by some chance due to some flight change it would come down to one in J and one in Y for international, we'd discuss it. But in view of my height and back and the fact that she's much shorter and much less spoiled than I am, I have to be honest that I'd opt for J unless she insisted otherwise (which I know she wouldn't).