You can't always use these words interchangeably. Examples:
1) comparative
Matti on vanhempi kuin Marko. [Matti is older than Marko.] (you can't say 'kuten Marko')
2) as ... as
Matti on yhtä pitkä kuin Marko. [Matti is as tall as Marko.] (you can't say 'kuten Marko')
3) If kuten means 'for example, among others', you can't use the word kuin
Hän myy marjoja, kuten mansikoita, mustikoita ja puolukoita. [He sells berries, for example strawberries, blueberries and lingonberries.]