Magic penetration IMO. Kat gets plenty of CDR whenever she gets a kill. All that CDR is really going to do late game is make it so that you only need 3 kills to reactivate DL instead of 4, while Mag Pen will make sure you'll get that kill.
If you go AP, maybe CDR is a better option, since you'll get plenty of Mag pen from having access to Abyssal Scepter/Void Staff. --- I reject your reality and substitute my own! -Adam Savage, The Mythbusters