A380 is IMHO the quietest, most spacious long-haul plane there is, especially in economy cabins.
Like other have said, if I am going to book a 20+ hour flight, I will specifically pick the flights that are A380 over either 777 or 747, and I simply would not consider anything narrow-bodied, if this means paying a bit more, so be it.
While I've never been in the economy cabin on a 380, so cannot directly compare Scuffer's experience, I totally agree, the noise level and the cabin altitude DO make a huge difference. So does the humidity, where the 747-8 is pretty good (only done one LH flight on this one, not many around). WRT to the 'noise-cancelling' headsets - every tried to sleep with one of them on? Good luck.