Bottom line: the 747-8 is the end of the 747 line. The A380-800 is the beginning of the A380 line. While you might have commonality with the 748F, there's no growth in the 747; once the 747-8 ceases production, that's it.
If you're going to make a major investment in a large pax aircraft (and it will be, if you're going to have a significant fleet), invest in something which will carry the airline at least 10-15 years into the future. Airbus will eventually develop a higher gross weight model for longer range, not to mention a stretched -900 model. On top of that, there is bound to be significant commonality between the A350 and A380.
For the last 30 years, the 747 has served CX superbly in various guises, just as many acft before it - the Electra, CV880, 707 and Tristar - have done; I have a huge admiration for the 747 and it will be a sad day when the last one rolls off the line, but time marches on CX needs the best bang for its buck.
That's the A380.