I think for most people, they should just play ap kat because it's hard to make AD worth it in the early game. People make a lot of mistakes and games stall out so the better scaling from ap is great. I have an item guide for ap kat in my 'how to play kat' playlist.
@Tomasz Knapik if you press e then f, katarina will flash first then instantly e when she in range. Normally pressing E just makes you walk towards the enemy champ until you are in range. Press Flash does not interrupt it so you can E>F to make the combo faster.
