Math for econ won't compensate for signalling of your math ability, since most people regard math courses taken in economics department as not being as rigorous as those in the math dept.
Grad Micro is a good signalling course, so it would work in your favor.
It's impossible to say how strong either will be relative to say poor performance in certain math courses. But it suffices to say that there will be applicants to at least top programs who would have performed well in both graduate econ and math courses. If you can't directly improve your math background, then i suppose this would be the next best alternative.
There are also many externalities involved with taking grad econ courses. The main one being access to strong letters of recommendation, so that's something to consider as well.