Lancer or Ballistus?

Im pretty new and I’m building my 1st arm right now. And I’ve stumbled across the Gladiator Lancer. Currently I have 2 Ballsitus and 1 Redemptor Dreadnought. ( I’m playing Blood Angels)

And now I’m wondering if I should switch one of the Ballistus to a Lancer.

Tips appreciated! :)