Is it better to choose recommenders by discipline or by familiarity?
Ideally you want someone who is well known to people on the hiring committee.

It sounds like you already have letters from two people who are very familiar with your work.  If you can get a STRONG letter from a "big shot" in the field (math?), you should go for it.  That person doesn't need to know you very well, but he/she should be able to put your work in context and explain why it is important/interesting.