jm62, Welcome to Flyertalk,
Since both awards are available and you have a lot of miles, I'd go for F. It isn't that many more miles (12,500) and it is a better flight experience. The issue between the two isn't a "bigger seat" but rather a lie flat seat vs. an angled seat.
However, you could do F on the outbound/overnight flight (so that you have actual lie flat seats and thus the potential for better sleep) and do J on the return.
Either F or J on the 777 are considerably better than F on the MD80 or 757, so no matter what you'll be much more comfortable.