TBH, given that you're currently happy with the school your DS is in, I don't quite see what's in it for you to follow this teacher. It sounds as though she's a good teacher, but it takes more than that to build a good school. Moreover, the fact that she's had some conflict at her current school has to raise a red flag that she may be more likely than the average teacher to have conflict at her new school too (possible for understandable reasons, but as you say, you have to consider the stability of the new situation you'd be moving to).
I think what I'd want to do would be
(a) to talk to someone senior at your current school about what this teacher does that is so fantastic and how you really hope those fantastic things will continue even though she's left, and how it was a difficult decision for you not to follow her;
(b) to explain to her that although you wish her well, you aren't willing to uproot your child from a good situation; but that you'd like to keep in touch and hear how the new school is going, in case you might want to move your child later.
