Will an undergraduate math or economics degree better position me for a top economics PhD program?
I've heard people say that your math education is more important than your economics education at the undergrad level if you're interested in going to grad school for econ.  Is this true? 


Yes, it is definitely true.  Almost everything in graduate level economics involves real analysis and optimisation conducted at quite a high level of proficiency.  Now, obviously there is a trade-off here, but generally students who come in with a solid mathematics education are at an advantage, even if this additional mathematical knowledge comes at the expense of an absence of previous exposure to economics.

One of the difficulties that most students encounter in graduate level economics courses is that they have to learn two things at once.  Most students struggle with the mathematics, and so they are trying to learn real analysis and various optimisation methods.  At the same time they are trying to learn the economic reasoning and intuition for the material they are studying.  Often the difficulty of the mathematics obscures economic understanding of the material, which creates learning problems.  If you are one of the lucky students who finds the mathematics easy, you will be able to turn all your brain-power to the task of understanding the economic principles and intuition of what you are being taught, and that is a major advantage.