They're the same car, literally. So it's just a matter of what style you like, as they use different cosmetics such as headlights, grills, some interior trim, etc... But take that makeup off and they're identical twins. Hyundai is the parent company of Kia.
They're both great cars, very competitive in that category. Reliable, good value, loaded with features. I personally like the Optima styling better, but that's subjective.