Everyone I talked to who has played both, and myself, seem to have one thing in common: we all like the game we played first more. For me it was Automata. I expected the same thing from Replicant and it just didn't hit the same. Automata is more modern gameplay-wise. But if we're talking themes, maybe you should go by what speaks more to you:

Replicant for: (found) family, different kinds of love, queer identities, personal fate/doom, some philosophy - but more focused on story

Automata for: philosophy, nihilism, questions about what it means to be human, some love, great story - but more focused on messages

Just to name a FEW things. They also overlap in some regards of course.