Math Minor in Math or Chemistry for ChemE?

I would recommend Chemistry.

I don't think employers will really look too closely at what your minor was after you finish your graduate degree. In my opinion it would be better to pick something that will benefit you in preparation for graduate school. There is a lot of overlap with your ChemE degree in both Chemistry and Math, so you'll already have most of both minors just by getting your ChemE degree. Rather than looking at Math minor vs Chem minor, look at what those few extra classes would be and ask yourself which is more relevant to your graduate program.

At least where I got my BS in Math from, most of the Math classes past Differential Equations are theoretical, and you'll take up through Differential Equations as an Engineering student anyway. What you'd get out of a Math minor would be some experience writing proofs, studying the theory of number systems, etc. It's more philosophical than practical. While that's really interesting stuff to study, I think additional chemistry courses would make you a better chemical engineer.