I honestly always prefered artifacts, since they give you theming bonus, which is culture and tourism, especially with artistry, but even without artistry, i kidna alwasy lack tourism, which is later very important, for
international trade route bonuses.
Landmarks are kinda "the long game", as they get most value in Information era, as u can build "Interpretative Center" which give you
"+1 Culture Culture and Tourism Tourism from all Landmarks in the Empire ".
But honestly i never did the exact math on this one. In terms of "instant boosts" landmarks give you more culture, and for the
"Influence wars" Culture is defence and Tourism is offence, so maybe after all culture is better, since you get progress towards policy.
But then again, the moment you have this considerations is so late into the game, it doesnt really matter.
Also, just noticed the bonus"
"50% of the Culture from World Wonders and Tiles is Converted into Tourism"
stack with hotels to 75%, and landmark with lets say 10+ cities will give you 20 culture later, which can be like 15 tourism.
But still even with hotel, 25% out of like 7-8 culture landmark could be like 1-2 Tourism.
At the same time Artifact give you if i recall correctly +3tourism +4culture (honestly dont remember if this changes with some buildings)
But then again if you have them in pairs, they give you theming bonus.
So i would guess inside your own teritory, go for Landmarks. Excess happiness also have some added benefit.
You have conversion to culture from fealty, each excess happiness give you 2% food growth, and you also can fight longer to mitogate
war weariness, or even resist Dissidents from other ideologies..
Also, you can always get more Great Works // Artifacts trought espeionage or conquering cities, but you cant print those Landmarks..
So i guess, as always, the answer is "it depends"

But i kinda convinced myselve, to get landmarks next time

I mean, id probably go Artifacts, if you have Artistry, since they boos theming bonus.. and you go artistry for tourism, so it kinda make sense.
Hmmm.. There were some bonuses based on the number of theming bonuses, i think it was Music, for expending Great Musician etc
Not sure if maybe artifacts increase Great writer Culture bomb?