If you play solo queue most of the time and intend on playing this champion in it, get Twitch. By no means is Kalista a solo queue champion.
Twitch plays like evelyn in the mid game and can carry a team really hard if in the right hands. Kalista relies on other people being comfortable with her ult and, unless someone else gets fed or builds properly, your team would have no way to deal with a fed ADC. Also, Kalista's apm:playability is much higher than other adcs and because of this, she has a high skill floor. (Twitch will carry harder, easier)
So basically, Kalista works well if you're playing with a team who's comfortable with her, and if you've practiced her a LOT. Twitch would be a saver pick if you intend on playing him/her in solo queue since he's more of a solo carry than Kalista. Get some nice ganks, put your team on your back, save christmas, win the olympics, and win the game. (and Kalista just can't do this)
Final note: Vayne is a strong duelist because her ultimate allows her to fight safely within an adc's range, without being auto-attacked. Only thing Kalista can do is jump around within the enemy adc's range, not really doing anything (kinda like the scene in Indiana Jones where the dude with the swords tried to intimidate Indy with flashy maneuvers, but he just shoots the guy)
Final Final note: If you just want a fun adc who's challenging to play, feel free to buy Kalista, but imo she has no place in solo queue, at least at this point in time.
FB: /ifmAries
YT: /tboi3000