1 - Take Pushy away from Veronica.
2 - All (human) companions are kill-stealers. Even their default weapons are still very strong they are AI-controlled aimbots who are far quicker/more accurate than you. That's just the way it is in this game.
Edit: Argh realized you were asking specifically about Cass or Veronica & I was giving an overall...sorry. Edited a bit.
That said....
Who is better for a guns character?I prefer companions to be ranged if I'm also ranged, because it means they don't get in my way as much. So Boone,
Cass, Raul, Arcade.
Who is better story wise?Subjective. They all have pretty decent stories/quests, just depends on what kind of chr./story arcs you like?
Who is more funny, has better dialogue?(Cass)
My votes would be Arcade, Cass, and Raul. I luvs him, but Boone does not talk much and dialogue-wise is not comedic.
Who is more useful (perks)?(Veronica)
Raul, Boone, ED-E, Arcade. imo.