I have taken pretty much every antidepressant there is and am currently on both Mirtazapine and Sertraline. Mirtazapine is indeed an antidepressant as its primary use and has good proven results for anxiety too. It really depends on personal tolerance and preference, everyone will react differently. Mirtazapine is known for weight gain which has been shown to be the main reason people stop taking it. It can also make you feel quite sedated for the first 4 or 5 weeks while you're getting used to it, but this goes once settles. Sertraline and Venlafaxine are more what I term as "uppers" compared to Mirtazapine or Amitriptyline for example. I have also taken Escitalopram and noticed no difference at all between that and Sertraline, or any of the SSRI class drugs. I've always had to start on smaller doses than the normal starting dose of any SSRI or SNRI, as otherwise they make me feel a bit ropey. But many many people don't need to do that, I'm just particularly sensitive (just know it's an option if you are too).
There really are pros and cons for each in terms of side effects and everyone's experience differs, so it's really a case of just finding what works best for you. Sertraline is my go-to medication for anxiety (but I have never suffered with depression) xx