Depends on your play style really. Do you like ap range casters or ad melee fighters? Irelia can be built either tanky dps, or attack speed with lifesteal. Kennen can go straight ap, or tank Kennen, or even ad Kennen (I suggest not ad kennen until you play several games with ap kennen first).
Both have really good farming capabilities, both have gap closers, both have stuns (Kennen has better stun potential though). Irelia has sustain right away though with her Hiten Style (true damage if you activate it for massive damage early on if you decide to trade auto attack blows). Kennen can easily get sustain with hextech revolver.
I can keep the list going and going, but really it just boils down to your play style: Do you like to rush into a team fight and cc them down while doing lots of magic damage? Or do you like to jump on the squishy champ and/or be able to do 1v3s (or higher) and be able to soak up a lot of damage?