@limabI would definitely prefer that you use the provided EPUB button.
The site's system creates a file the first time somebody downloads one and caches it. It keeps it updated as necessary when the text is changed or new text is added.
Using FFF puts undue load on the site's servers. It doesn't use compression, so it uses too much bandwidth, it's not cached so the files need to be generated constantly.
For a story that has 200 chapters, the EPUB button takes one full transaction on the server to send you the file. It looks up the story, looks up your account, then sends you the file and then updates your counters and the story's counters and the stats counters.
FFF will do at least 201 (if configured to do strictly html files) full transactions and the transactions are bigger as it has to also fetch each chapter's text, all at full bandwidth.
For example, Caleb by pastmaster has 87 chapters and is over 5.2 MB in text. To download the EPUB file it's about 8 Queries to the database and 2.1 MB of bandwidth.
To Download it using FFF, it's about 1050 queries to the database and 5.5 MB of bandwidth.
8 Queries vs 1000+, so at least double the bandwidth and two orders of magnitude more database queries.
So, EPUB file.