Good PDFs are designed to look great at a single page size. Everything fits on the page, the paragraphs are broken at the right places, there are no widows or orphans, illustrations or tables appear where they are needed, vertical spacing is adjusted to make the page look right, etc.
Good ePUBs are made to look good at every (most) page sizes, font sizes, etc. The page won't look "perfect", but it will be good enough and, more important, it will look equally good if you cange the size, the preferences, etc. And you could even disable illustrations
For your own use (fixed parameters), there is nothing you can do with ePUB that you cannot do with PDF, and there are things you can do with PDF that are not possible with ePUB.
As soon as you send the files to others, since they'll have different tastes, needs, devices, etc., there are things they can do to the ePUB that they cannot do to the PDF.