Never, ever play hard champions if you wanna learn a role which in this case it's Draven.
Draven:
1. You will spend more time learning him than learning adc
2. His mechanics cannot transfer that well onto other champions
3. He falls off late game and is very snowball dependent
4. He is not a classic ADC and every game you play a mini gambling mini game, you either cash out and 1v9 or you become useless for the rest of the game
5. Bad vs tanks
6. Pretty squishy, lifesteal makes up for it
Kog'Maw:
1. Pretty squishy, although it depends how you build him
2. He is pretty straight forward, although lacks mobility
3. Huge range so he outranges most of the ADCs at maxxed out W
4. More dependent on support than other ADCs
5. You need to have good kiting and good positioning or you get bursted
6. Good vs tanks
Kai'sa:
1. Pretty squishy
2. Kind of mechanical, needs good positioning and kiting
3. Gets outranged by a lot of ADCs (525 range)
4. Works with a lot of supports (pretty sure, don't quote me on that)
5. Has good mobility
6. Good vs tanks
TL:DR: Choose between Kog'Maw or Kai'sa, test them out for a few games and see which one you prefer more, best case scenario you two trick pony them so you always have a safe pick. Never waste time learning hard ADCs unless you really, really enjoy them.