Should I vote for Q to join or let Jax to join us get the flintlock pistols on Ruben's study?

If you choose Q, he goes with you. Not sure about what happens with Jax. You have a later choice about whether to give the pistol to Ruben or Q so perhaps that is relevant to consider if you choose Q or Jax.

Q with the pistol freezes and does not shoot Margot but it appears like no one else gets hurt in that immediate situation. Carla tazes Margot and Ruben is not shot.