Please understand that I own both a Kia and Hyundai and like them both, so I am not biased towards one or the other. But what I see from industry sources is:
*Up until now, Hyundai has run circles around Kia in terms of quality. Even amongst similar vehicles, the Hyundai has always had the edge. It helps that Hyundai hasn't sold a real quality lemon in a while. The 2002-05 Kia Sedona really drug down Kia's quality (admitting this even though i own one).*From the people who test-drive these vehicles, the closest consistent comparison given is that Hyundai tends to tune their vehicles a little "sportier." I haven't noticed that at all, but if so, it means that Kia is a lot like a Pontiac, or at least what the Pontiac moniker is supposed to mean. Hyundai is trying to change its image from Chevrolet to Oldsmobile.
But, like GM, the Hyundai and Kia vehicles are becoming so similar that I expect any differences to be pretty much gone within the next couple of years.