I would do J round trip, because you have no previous experience of CX J or F, you will likely be extremely satisfied already with J. You will have comfort both ways instead of one way. Also if you fly via ORD, you have a chance of flying CX new J product.

Whilst flying CX F is amazing, you will have to fly Y one way. Personally I would dislike mixing a longhaul trip between Y and a premium class, Id rather settle for medium to good comfort in J for both segments so that I was in a premium class for both flights. Both flight segments will be long, so its nice to have a lie flat bed for both flights. Whilst food is nowhere near F standard, service should still be excellent.