I'd have to say the story of Romeo and Juliet is the more tragic tale. At least Beren and Luthien were able to return to life and have a son.
Shakespeare's story is the more tragic as Beren and Lúthien ultimately get a happy ending.
Undoubtedly, it has to be Romeo and Juliet as the more tragic tale! Beren may have died but ultimately he was taken pity upon by Mandos, and was allowed to return and live a mortal life with Luthien. No such luck for Romeo and Juliet! Both classic tales but very different endings!😍🙂