Kalista Double ADC Bot Lane
My personal opinion is, Kalista is better as support.
Her ult is Zilean tier + allows you to reposition. (Not to mention the awesome knockup from afar if you're ranged) Her W scales %health and not items. + Its magic damage. Her E acts as a second (BETTER) smite and slows the enemy. She has good mobility to survive ganks and to reposition herself away from incoming skillshots. She has free wards with her W (Perfect for low elo that refuse to admit warding is necessary)
Playing Kalista ADC, you'll find its pretty difficult to harass with W proc on a consistent basis if your supp isn't ranged.
Saving the life of your ADC is much more important than either saving your support- or using them unreliably to knock up enemies.
Just my two cents though, I know its been mentioned, but Kalista in soloQ is very unreliable as an ADC. I do think she's a VERY potent support if your team already has some CC + tankiness though.
CONS---- Her W procs can give the kills to her. (Most likely not, because it procs the first time and you let your ADC secure the kill) --- //An early sightstone never hurt anyone though She is fragile and has less CC than some of the popular supports. She has other notable cons but losing bot lane is hardly one of them.
Try it out with Twitch, you won't regret it.
If you've tried it or have any thoughts be sure to chime in.