Kog Maw or Caitlyn

Caitlyn... duh

Decide what you want to do as an ADC. Those champions are picked for entirely different purposes.

Generally speaking, Jinx and Vayne cover the long range auto attacker and tank buster niches better than Koggy can cover the both.

Cait is great for a relatively easy to grasp ADC who trades raw damage for safety.

Also, Cait is getting a light rework with the ADC pass coming up soon. To my knowledge, Koggy is only getting a few number adjustments.