a few things:

1.) just an fyi, CX doesn't fly to SEA. So it sounds like your choices for CX metal are SFO/LAX (or perhaps YVR?) to SIN. One of the YVR flights daily has F.

2.) You might want to do some searching around here first. There are differences - some might say minor, others like me say major - between J and F. They've been debated endlessly on this thread...your question touches on perhaps the most-discussed Cathay FT forum topics, including a few identical threads! For tangential info check out the menu threads, meals, debates between 747 and 777 (both discuss a lot about J vs F differences), F on 777 vs 747, etc. The FT search function isn't as handy as google but if you fiddle with it enough using the advanced function you'll get what you're looking for.