Yeah, Harvard is a strong brand and I'm not well-versed in the field of international environmental development. To me, it might depend on where you want to work after graduation. If you want to live and work in the Chicago area, then I would still say University of Chicago tops Harvard. If you want to get into the broader ID community (UN, World Bank, etc.), the Harvard name will get you the better opportunities off the bat.
Also, I think you should consider what type of work you would like to do. If you want to do policy analysis, then University of Chicago is probably the stronger of the two, heavy in quantitative analysis and research. If you want to administer the programmatic side of things, then Harvard is likely a better fit, as it incorporates management and leadership training.
There is a definite case to be made for University of Chicago, but if you are unsure of your path then greendiplomat might be right.