you are totally right with your considerations. i'm working on matter of life and dear right now, it seems to be better in terms of sands of time and ironite drops, but that's only a feeling.
only your gold argument is in my opinion neglectable at the moment, because the most endgame players sit on millions and millions of gold and have no idea how to spend it. that's a common endgame problem, that's
why i think we can neglect the gold factor.
so far it seems, that the droprate of 3* runes is a little lower than on underworld madness. but that's also still a guess, because the numbers are very small.
i will also try kingdoms of sands, but unless someone wants to buy me ironite it will take some time :-D i belive the 3* droprates are much lower, but this is only gut feeling. will try to verificate/falsificate that any time soon...
First observations matter of life and dead madness: