My own personal view is that Xander falls into the inessential category with Oz, despite all the protest that he represents "heart" and is therefore important. He gets in the way at least as often as he helps, but what can you say, he manages to be loveable despite all of his many flaws, so we forgive him and appreciate his presence. Oz doesn't contribute much himself but I feel that if we are willing to say Tara helps somehow to calm people down, I think it's fair to say that Oz serves to accomplish a similar effect. Oz's presence can definitely be calming and grounding, if also inessential. I don't give Tara any more credit though. Some would call her influence calming, and at times I would agree she can be, but she can also (often) be the source of awkward silence, and she's a bit of a wallflower too. There are many scenes (with her in them) where her presence completely vanishes, even after the episode "Family". Her presence isn't assertive enough to be felt in a large number of scenes. One sometimes forgets she's even there.
Of the two though, I would take Tara over Oz. Tara's relationship with Willow is where she really shines. Her introverted nature blossoms when she's alone with Willow, and she brings out features and aspects of Willow's character that otherwise go unstimulated in literally anyone else's company. That includes Oz. In the balance, I would say this is mostly for the better.