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I’ve flown both, most recently the A340, and each has advantages.
Being in the nose of the 747 is an experience but the cabin is a bit more concentrated.
On the A340 it feels a bit more open.
The seat on the 747-8 is a bit newer too.
On the 747 the cabin is a bit more exclusive as it is separated from the other cabins.
On the A340 they were boarding J passengers through the F cabin to speed things up.