I think the biggest difference between F and J on the transcon 321T is the "seatmate" configuration. J window seats do not have direct aisle access, so J window seat pax have to climb over sleeping J aisle seatmates to get to the bathroom.
That said, both J and F have lie flat seating and, in my experience, similar food and drink.
I should also observe that F seats are cheaper than J seats with some frequency due, I suspect, to corporate flyer guidelines that forbid F seats when J seats are available.