Honestly, I don't know. It's a bit mixed up. Kia seems to offer more 'youthful' and sporty vehicles in the lower price range (Soul, Forte SX, Forte Koup SX) while Hyundai is really conservative in the same price range (Accent, Elantra). Then Hyundai has the Genesis coupe for the enthusiast and the Genesis sedan and upcoming Equus for the luxury market.
It seems to me that Hyundai is trying to cover all of the market, but plays it too conservative, leaving a lot of room for Kia to supplant it in certain brackets.
I much prefer the Rio, Forte, Magentis*, Sportage & Sorento over their current direct equivalents over at Hyundai (*over the current Sonata, that is. It'll most likely change when the new Sonata is here.) And that leaves the Soul & Rondo without challenge from Hyundai, again showing Kia as the more exciting/young family type of company than Hyundai.
I think Hyundai will have a problem with Kia at some point if they can't get out of their way soon. But then again, as long as their are big enough differences in styling and content in both brands, maybe it's not that big of an issue.