I've just bought the Hyundai ix35, so recently that I yet to collect it . I too couldn't decide however the top of the range Hyundai has a better spec and managed to get an ex demonstrator with all the extras already added.
Kia is part-owned by Hyundai, the Sportage shares a platform with the ix35 (the previous generation did at any rate, no reason to believe they haven't been developed in parallel) and it uses Hyundai engines. They're basically the same car.
Go with whichever one looks the best to you, or which you can get the best deal on.