Completely agree with Silburfuchs. Why not get the rating and time in the Whale before it is phased out on the pax side? You can always fly and enjoy the 777 for the rest of your career if you wish.
Who cares about the limited layovers on the 400/800 - again, you will get to most of those destinations in time on the other fleets. Regardless, I would absolutely want flight time in the 800F - but that is just me. Plus, as an SO, you won't get much actual flying anyway - you will either sit on the 777 or on the 747. But having that experience on the 747 will always be valued and memorable IMHO. Every 747 pilot I know valued that experience and that aircraft. You can always reflect back on that experience on the 747 as you skipper a 777 or A350 later in life.
I certainly don't fly for Cathay, but if I were in your position and if I had a choice, I'd personally would go with the Whale and enjoy that experience while it lasted...
