Using subdomains will keep the most link juice for your original articles. Keeping all the pages on the same domain will spread more link juice to the translated pages.
In the old days, Google would rank pages that appeared to be in the "root directory" higher that pages that appeared to be in "subdirectories." Strangely, I am now seeing extensive examples showing that Google has completely reversed that preference. Pages that appear to be in subdirectories are now receiving a noticeable preference.