Teemo is more broken for sure.
Katarina has SOOOO much counterplay. She has to channel her ultimate in one place - so easy to interrupt it and immediately shut her down. It takes very little skill to actually outplay a Katarina. Even a simple exhaust can let your team shut her out of a fight.
Teemo on the other hand? Dude doesn't even need to be in a fight. He can sit at the fountain while his team fights and loses a 5v4 4 kills to 0. Then put that little demonic laugh on display as the enemy team attempts to run towards baron after winning their fight and joyfully watch as they step on a million teemo-shrooms and he gets a pentakill while sitting at the fountain!
Caveat: I actually think Katarina is somewhat weak right now (and she is getting buffs on the PBE, which is unfortunate because she isn't a really fun champion to play against and I'd rather have her be slightly subpar). Teemo on the other hand is fairly balanced and possibly slightly strong on the right matchups. He doesn't fair well in this meta since he isn't tanky at all, but his shrooms provide loads of utility that will always be very useful. He just needs some teamwork for his allies to lure enemies in to his shroom-pits-of-death.