Actually, Singapore Airlines is watching how well the A380-800 does when it comes to still-air range at standard mean takeoff weight (MTOW). If the A380-800 can demonstrate the original 8,000 nautical mile still-air range at standard MTOW, then Singapore Airlines would end up buying more A380-800's than buying the 747-8I. However, the air cargo division of Singapore Airlines would definitely want to buy the 747-8F, though.
I believe that besides Qantas (which will use the 747-8I on the Sydney-Dallas and Sydney-Johannesberg routes), the primary customers will be Japan Airlines and British Airways, since both of these airlines are least affected by landing slot restrictions at their own home airports.
Also Lufthansa airlines, I believe it was Lufthansa airlines who have been telling Boeing to build a large 747 for the passed 10 years.