Kalista's E is currently way stronger than Twitch's E. It stacks infinitely, has a super strong slow, its cooldown resets on kill and refunds mana, and it does a lot more damage.
Maybe Twitch's E could do something special like steal AD from enemies for a short period of time.
EDIT: Wow, this got a lot of attention! Thank you guys! And for you saying that Twitch has a trade off in that he does true damage over time, the maximum damage it does is 216 later in the game.