Are there enough faculty with research you are interested in at both? Which location do you prefer?
Is money an issue for you, because Chicago is expensive?
Which school felt more like a fit on your visit?
Endofevagelion, feel free to PM me. I am UIowa MSTP student. I would be happy to help you in anyway possible. While I will be biased towards Iowa, I will be sure to give you honest answers.
Regardless, good luck and congrats on having two great options.