Certainly on most RTW tickets I think the difference is worth it. Mrs christep and I are currently in the middle of AONE3s ex-MNL where the increment for F was $2300-ish ($6700 vs $9000 incl taxes etc through AA). For me that's worth it for F rather than J on HKG-JFK, LAX-LHR, LHR-DXB-LHR, LHR-HKG, HKG-NRT-HKG (and MNL-HKG, HKG-MNL, though no real value there).