Both types have Fi as their auxiliary (secondary) function, but in the case of the ESFP that Fi is steering the dominant function of Se. In the case of the ENFP, that Fi is steering the dominant function of Ne. This makes for a big difference, as does the fact that the ESFP is of the Keirseyan SP temperament, while the ENFP is an NF. Hmm, I will have to come back later and give more concrete examples for this. In the meantime, reading about the SP and NF temperaments will probably help a lot. Here's Keirsey's site: Personality Test - Keirsey Temperament Website. Click on "the four temperaments" at the top of the page.
Hope this helps!