What minor would compliment a sustainability major more, economics or political science?

After first having to look up what a sustainability major is, I would lean toward Political Science.  Sustainability seems distantly related to the old Urban Planning major, but with both urban and rural facets.  An understanding of the governmental units and functions involved in such projects would seem helpful. 

Kinda depends on the job you want, right?  I have no idea what a "sustainability major" includes or what skills and knowledge it would provide -- or not. Are you going to focus on analytical skills, business skills, activism, or engineering?  Do you want a job at an energy company, a manufacture, a law office?

Start by looking up job titles and descriptions on a website like Indeed. Find out what a corporate sustainability strategist, energy consultant, or recycling plant supervisor needs to know.  What classes would you need to take in addition to your major to have the background to qualify for the job you might want?

You don't have to do a minor, but you do want to fill out our academic and practical experience with abilities you'll need to get the job you want. 

https://sustain.wisconsin.edu/blog/sustainability-...

if you're serious, I suggest economics.  Far too many politicians already have no idea of how and economy actually works and thus make wild pronouncements of pie in the sky dreams [and no body believes them]