Normally, questions like the one asked in the title are easy to answer: "Whatever is published first and which is in the original language." But when it comes to this interesting anthology that Jorge Luis Borges, Adolfo Bioy Casares, and Silvina Ocampo put together in 1940 (and expanded in the 1960s), the lines become a bit blurred. Look at the picture of the partial table of contents for both the English and Spanish editions:
If the reader who owns both anthologies, as I obviously do, wants to read as many stories in "the original" as possible, to which version does s/he turn? Although there certainly are translations in both that are neither Spanish nor English in origin, the majority of these tales come from writers in those two languages. If the percentages are nearly equal (and they are not, as 42 stories were originally written in English, 15 in Spanish), do both have equally valid claims for being the anthology "in the original?" What about the numbers I just provided? The Spanish edition came first, with the English not being published until 1988, if I understand the copyright correctly. Yet the English edition has by far the great number of stories "in the original."
Or does "reading it in the original" cover more than just the idiom of choice for the original compositions? Is there something to be said for reading stories that Borges, Bioy Casares, and Ocampo translated into Spanish (there are likely other, unlisted translators from which they selected several of their tales from non-English sources)? There certainly could be a strong argument for viewing the impact of the anthology as a whole from the viewpoint of those experiencing these "foreign" tales in the same idiom as the original Spanish stories. But others may argue, with certain evidence to support them, that to experience the stories best, that one should encounter them in their original idiom, which in this case is English.
It certainly is an intriguing issue to consider, which is why I'm blogging this little bit, but one that could be solved for a few by just sampling from each and then cross-reading them in the English/Spanish translations and then evaluating the anthology's impact in each language.