[ZodWallop]

TL;DR: Both are nice. You really can't go wrong with either.

If you'd like to read too much info on each, please proceed:

ePubs and kepubs use different layout engines. ePubs use RMDSK and kepubs use access. Each has strengths and weaknesses.

I use kepubs primarily because of the way page numbering works. If a chapter says it is 22 pages, then you turn the page 22 times and you will be at the end of the chapter. If you enlarge the font so now fewer words fit on the screen, the chapter length will change to reflect that.

EPub uses some formula, so a page can be anything from less than a screen to three screens. So a 22 page chapter could mean you would be turning the page anywhere from 20 to 60 times. But the page count will stay the same no matter how large or how small the font is.

This used to matter when I was in the office and would read on fifteen minute breaks. Kepubs page count felt more accurate to me.

Since I've been working from home and time isn't so regimented, I've been switching between reading kepubs on my Kobo and ePubs on my Nook.

ePubs tend to have better layout. Kepubs will sometimes leave big gaps between words when the text is justified. Fonts also tend to look better on ePub.

The pop-up footnotes are nice, but if you don't read a lot of fiction with footnotes, you won't use it much.

If you are giving these to relatives, I would say use kepubs. The most common question from people new to ereaders (that handle epubs) is why the page number doesn't change when they turn the page.