It truly is subjective. Reading some of the above I/we find that, yes there is more space in First and the service is, (or at least should - depends on the crew), be better BUT if you don’t have Gold/Emerald/CCR status then the lounge experience and check in is pretty much worth the extra miles, although it isn’t worth the extra money!

Things have changed a bit now that the spa is no longer open but overall we are perfectly happy in J on the plane but the F ground experience is much better.

So overall if you are Gold/CCR I’d say go for J and if not go for F.

Edited: just after posting I realised this pretty much related to LHR and also JFK and you didn’t mention where, only that it was a return leg so check what you get on the ground!