Obviously, an A380 is a lot more fuel efficient compared to a 747-400, it's much newer technology after all.
In terms of A380 versus 747-800, a war has been fought by Boeing and Airbus over this, which each company purporting their design is best.
In the end, I found the most credible opinion on this
that what Lufthansa was voicing earlier this year and which shows that
fuel flow on the 747-8 is "more than 10%" lower than the 747-400 at the same weight, and its efficiency is close to the much larger A380 in Lufthansa's fleet, in seat/mile cost terms: "We believe we will get very close - not quite [equal] - to the A380.
That sounds quite credible to me - the A380 and 747-800 quite close, with the A380 overall slightly better. Certainly sounds more believable than the marketing speak from both Boeing and Airbus.
