I had not noticed the difference in fonts - one difference is on page numbering: total number of pages with epub rendere, chapter screen turns on Kepub. The difference I personally like best is that footnotes can be previewed as pop-ups with the kepub renderer (but not with the epub renderer). I think firmware versions also change the differences. There is a
kepub.epub thread with differences summarised from
this post onwards, but it is quite old, so some of them may no longer apply.