I'd say the writing of TitS is far, far superior to Xenosaga. It won't win any Oscars, but TitS has a... straightforward, simple and delightful writing imo, that captures the essence of its main story, which is a coming-of-age roadtrip/adventure of two siblings in a muted, fantasy world.
Its writing, imo, is very character-driven first-and-foremost, and you learn about the world through the myriad amount of interaction you derive from the constantly changing NPC dialogue that each tell their own personal story within that setting. It's a rich world that slowly unravels itself through your effort to learn more about it.
I would say Xenosaga's problem in its writing is that it tends to be overly convoluted, and occasionally doesn't know where to put its characterisation in alignment with the storytelling. It tries to be character-focused, but it allows the mumbo-jumbo of the lore to creep into every aspect of the character to the point that it felt unreal, especially since Xenosaga tends to go the very deep end of scifi fantasy.