777 will have you dried up like a prune by the time you get to london! LOL! But also a smaller more modern cabins so makes it seem less crowded when full.
747 has four engines but tends to be high capacity version on the BKK/SYD route. Mind you the 747's are currently getting the AVOD system in line with the new club world and to my knowledge none of the 777's have been upgraded so this may be your better bet!