When in neutral should I be using f.S over 2S and vice-versa?

You should be using 2S most of the time in neutral to stop people on the ground. They're both 9f startup with similar recovery but 2S has a decent vertical hitbox but more importantly doesn't get low profiled by things like sol 2D, chipp's alpha blade, or some character's 6P. 5S is more about maintaining an advantage and keeping people from attempting to move or jump away. In one specific instance, you will also want to use 5S instead of 2S to punish chipp's 2nd rekka on IB as if chipp does the 3rd follow-up it'll jump 2S.