“Umass works out to be a bit cheaper.”
Check the numbers here: http://www.finaid.org/calculators/awardletteradvanced.phtml
The overall cost of living (off-campus housing, food, etc.) is probably slightly lower in Iowa City. However, transportation costs for you may be somewhat lower to Amherst. Don’t forget to investigate those. Also, be sure to find out what you need to do in order to keep your scholarships at each place.
Both of these universities can get you into grad school. Don’t worry about that. Iowa City is a lively community, but it is a lot farther from big cities than Amherst is. Getting anywhere from there will take a lot more planning than it would from Amherst.
What you mean by social life and diversity can be very different from what someone else would mean. I’d suggest that you contact the International Students Office at each campus. They can give you better inside information than most of us can.