At the moment, neither Caitlyn nor Varus are super top tier picks, it seems, but Caitlyn is more popular at higher levels than Varus, not sure how much of that is due to power or preference, though. Both are very strong if played correctly, though. Between Cait and Varus, I feel like Varus is more like Twitch, since Varus detonates his blight stacks sort of like how Twitch has stacks of his poison. I feel like Varus is better at poking since his Q direction can be less telegraphed than Caitlyn's; Caitlyn is more about controlling a lane with her traps and bullying the other laner out (she has a harder time in mid-game/teamfights if she doesn't get ahead of the other laner). Varus has a harder time currently simply because he has less mobility, but if you're good with Twitch (who also has very little defensive ability, since his stealth takes so long to activate when he's taking damage, and he only has a slow), Varus shouldn't be much different, I think.
Maybe also give Ezreal or Corki a try? Both are good mid-game ADCs that can safely poke from range. I'm personally waiting for Kalista to come out, she might be your best bet since her E functions a lot like Twitch's E, and she has ranged poke with Q, but she's still pretty new (and still in PBE) so not sure how she'll scale into teamfights. Like Tanukihero said, Jinx is also a great hypercarry with good poke with Fishbones and Zap!, and, like Twitch, if she can stay safe in a teamfight, she'll do some serious damage.