So i love Zaun and i need help choosing a champion. So i love long range, going mid and stealthing. So which champion is stronger twitch or Viktor

They're not even close to similar, all they have in common is that their auto attacks are ranged. Have you actually tried them out? If not, wait until you do. (Also, Twitch is getting a rework, so I'd wait until you try out his new incarnation regardless.)

It doesn't matter who is Bob thinks is stronger, because if you don't enjoy them, either you won't play them or if you do, you won't enjoy the game.

They are two different champions, so comparing won't help you much. Viktor is an AP mid, while Twitch is an AD carry. With Twitch's rework, I have no idea what would happen. With Viktor, he is difficult to play. Viktor has trouble escaping from fights..since all he has is a stun pad. Twitch has trouble with Visions or Oracles is gonna make the game tougher. Who you like better is your call, and each have a different playing style.

Twitch is a good assassin and if built probably can be a huge stealth carry late game. Viktor is an AP harass and CC champion that could also deal massive damage if built properly.

Both Champions require extreme positioning and Twitch even more now. This will depend more on your skills than who to buy.