A combination of both. Some games I quit because I couldn't get past how bad it looks, and others I had difficulty finishing because the plot was weird.
If i have to choose - then graphics
But usually quite neither of them in avns; for me you have to see some combination of the plot twist or setting with some uniqueness to the characters
e.g. Insexual Awakening has shitty graphics and basic plot but there's something that makes u play it while there are a ton of good rendered but generic big boob bimbos in every game