IMO, the F seat is considerably -massively, actually - more comfortable than the J. It's true that the F seating is slightly awkward for a couple, and I would rather sit at the window than in two middle seats-- still, these issues still pale in comparison to the fantastic F seats in which I can sleep 10x better than in J. The only time I'd consider J over F is traveling with Mrs. KFM and Bay KFM so we could flank the kid and also provide a little less baby to our fellow passengers...
Go for it. I'd be stunned if you found the extra miles to be anything but well-spent. The F seat on the 777 is unquestionably the best seat in the entire AA fleet-- might as well experience the top banana...