I don't have a preference for either. Both can be good depending on the actual art. I would take an average 2dcg art over an average 3dcg render though.
I guess 3Dcg - I follow a few 2D games but now that I think about it they all have a pretty high threshold for art quality, I'm definitely more forgiving of low quality art if it's 3D.
Compare Eternum to any 2D game and my answer is 3D only. 2D are annoying, they are too much like old Nintendo games and the nostalgia for those is long gone.
As long as the game is well polished and the author has put in the required effort - it does not matter to me.
Consistent art style is important.
But stock DAZ-models without good lighting is a turnoff - and that is unfortunately a lot of renpy-games.
If we talk about GOOD art, I prefer 2D, but not by much, but more than anything because 2D has versatility, I mean that there are many different styles so there is variety to choose from, 3D on the other hand, as a rule, is almost always dependent on the degree of realism that artists make, there are not so many styles.