Actually, I think neither - compared to Ico or Shadow of the Colossus, The Last Guardian seems to have a much lighter, more innocent tone to it - compared to the desperate escape of Ico, and the solemn and gruelling quest of SoTC, TLG seems almost like a simple minded adventure between a boy and his companion.
Considering this in mind, this game might actually have a
happy ending to it, where the boy and the gryphon are ok!
Dark and moody is
so 2000's - its a new decade, and I think that its time for light and happy to make a return
edit: just mentioning guys, I've never actually played either game (sadly...), so my judgement is purely on what I've heard about each game. I do hope to play them once the Collection edition comes out!