Vi or Darius?

Yes

When in doubt, I ask myself the following: Which champ would better expand my champ pool? As in, would you benefit more from having a jungler or a top laner? Yeah, they both can technically play both roles, but not optimally.

If that doesn't help try these: Who's better in the meta? Vi. Which is more fun? That's up to you.

Short answer: Vi is better, but banned often.

I have yet to play Vi, but I have recently purchased Darius and so far I really like him.

Despite of all the negative stigma around him, (OP, players being jerks, etc.) I think he is a fun champ to play..

Are there some better as far as a Tank and off-spec fighter goes? Sure.. But for me I enjoy Darius.

But its all a matter of personal choice.

My worse purchase was Gragas.... I can't stand him... lol

Darius is fun to play and unfun to play against. Also useless, if he is not super ahead. Gets kited. Vi is an assasin that builds tank and still deletes people with her combo... She is useful even when behind, unlike Darius; just don't ult away from your team, if noone is following you.