SL wasn't difficult at all.
Neither of them are difficult in my eyes.
So I'll rate by how entertaining they were.
CoO was much more enjoyable to me.
I love fighting all those Darknuts. (I can do it without taking any hits, and no, I'm not sniping them.) It's the only place end game where I can get some real combat going.
Beating SL was meh to me, didn't feel like I accomplished anything.
But CoO, everytime I went in there and cleared it without taking a hit, fighting all those Darknuts at the end... I don't think I've ever felt that exhilirated playing any game.

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πάντα άγρυπνος
As all gods are of human creation, worshipping an external god is to worship another human by proxy

Ангелы и демоны кружили надо мной
Рассекали тернии и млечные пути
Не знает счастья только тот,
Кто его зова понять не смог...