Vpod versus Freight
B747 engine can only be freighted on a DC10 or MD 11 or B747 main deck freighter, its too big for lower lobe hold of either. Freighter availablity is the main factor, much quicker turnaround when freighted but extremly expensive south bound. VPod saves time if freighter not immediately available going to where engine change is required. QF always want aircraft back in service ASAP.
A380 also requires a B747 main deck freighter to ship a spare engine. All QF A380 engines are shipped on SQ main deck freighter from new and overhaul.