Hey, I'm actually planning on focusing and mastering Kennen
Both AP and AS/AD are good styles for Kennen, depending on who you are matching up against
It also depends on your playing style, whether you want burst AoE damage or DPS single target damage
AP style for Kennen is more like a get in and get out damage output and is substantially better in harassing with the shuriken, having more damage
AS/AD style can be tough to master since you have to be careful not to die early and farm gear as fast as you can, and once you do get your gear and being a ninja, the other team is doomed; even capable of killing TF in a 1v1 fight using stun lock
Lastly, you just have to observe who you are facing up against
For me usually when the majority of my opponent's team are tanks, I prefer AS/AD
And if the majority of the other team are mages or carry champs, I prefer AP
Hope this information helps you decide
I myself chose to focus on AS/AD Kennen because it's more fun and different
Maybe I'll make a guide for Kennen one day...
P.S.: From experience, many players look down upon AS/AD because it takes time to be of use to the team, and they want this burst AoE damage from Kennen