From a historical point of view, the "ecash.pdf" file is older than the "bitcoin.pdf".
Actually, we can't be sure of the text inside. Because this is not an original.
BUT!!! I have a good new. I found a october 3, 2008 version! The one i was searching for! (but an original ecash.pdf can be older)
HERE is the rare 2008 version : https://www.gwern.net/docs/bitcoin/20081003-nakamoto-bitcoindraft.pdf
After a quick study, i can tell u there is small but existing differences in the text, between 2008 and 2009 documents.
And in the structure, 2008 pdf was created on a UNIX system, and the 2009, on a Mac. Interesting!