The only thing where SL in better than TFTUV is the dynamics between the MC and his LIs. In SL he mainly concentrates on his family plus some side chicks, but in TFTUV it's all over the place. The SL mostly concentrates on the lewds and TFTUV on the story, so the sexual content in the SL seems to be more plentiful and concentrated. I find Adriana and Sasha from the SL to be among the sexiest girls from the games I played so far and TFTUV has nothing of their kind.While Sisterly Lust is quite dated from an aesthetic standpoint, his current game, Tales From the Unending Void, is *much* better visually, and there is still plenty of incest if that is what you're looking for.
To put it short, as the whole product TFTUV is miles ahead but for description of the lewds and love dynamics I prefer SL as for me the text is at least as powerful as the images.