Outbound, if only because you can use the CCR at Heathrow. I doubt SEA has anything special for F pax. I would also always assume, in general, that food is better on flights ex-London because BA has more control over what is loaded.

And it is always better to start your holiday in style. By the time you fly back, you are tired out and already worrying about what is in store when you return.

Only reason not to do F out is if it is an all-night night flight and the return was an all-day flight, but that is unlikely from the US.

And as Blue, of course, you can seat select in F so you can be certain to be together. There is a tiny chance in J you will be separated as you can't seat select until you check-in. If this is going to happen, its better on the return and certainly better overnight.