Talon, like most assassins, are a niche pick.
Typically they are picked when the enemy has a high priority target that needs to be dealt with quickly.
As such, Talon can get to said carry while Viktor can't. In certain instances he can kill faster than Viktor, get fed harder than Viktor, and if you give him a few tanky items he lives long enough to do his job and then get out.
However, if you are taking an AD champion mid, it is usually best to have an AP top laner, otherwise your enemy will stack armor and your whole team's damage becomes useless.
As for Viktor, like most mages he has respectable burst and is generally more useful in a teamfight.
The problem with Viktor is that his CC is on a delay, and he is usually a prime target for an assassin.
So, to recap.
Viktor is a better generalist pick for a team.
However if your top laner goes AP, you can safely go Talon without jeprodizing your team's damage output.
If you play Talon, remember to roam bot when you can to score those juicy double kills.