There are several differences between the K9 and the E9. The E9's finish is not as nice as the K9. The front sight of the E9 is pinned while the K9 is dovetailed. The grips are hard plastic on the E9 while the K9 has soft grips. As has been pointed out, the K9 has some extra machine work done on the slide (mostly rounding off edges, sharp areas). Generally the E9 comes standard with just one magazine while the K9 has two. Other than that they are basically the same firearm function-wise.
All that being said, the E9 is a great value if you get a good price on it. Yes, the K9 is somewhat more refined, but I would guess you wouldn't notice a bit of difference in how they shoot. The E9 also comes with Kahr's warranty same as the K9. If you were to ever have a problem just ship it back and they should fix it. Kahr's are great guns and you should enjoy it for a long time.
Shake