Major metros will make you colder and slower to trust strangers, but you'll know a lot more about the world. There's simply more variance in cities. Living your life in a backwater town won't disabuse you of being trusting and friendly, but your preconceptions about the rest of the world won't be tested either.
Travel to other countries is better than either to broaden perspectives. Seeing the world from multiple points of view (sometimes literally) is the best way to eliminate bias.
I strongly disagree that metros will make you know "more about the world." They will let you know more about a specific bubble that is just as narrow as the bubble in any small town. You can travel the world and run into people who believe in exactly the same things you do and have read the same books and seen the same movies. In fact, it's hard not to.
I remember when I was a grad student, they would pair me up with a different foreign student each year. They were all cookie cutters of each other. One from Germany, one from the UK, one from Spain. They all immediately came here and complained about U.S. gun control and how the U.S. was too religious (Bible belters!) and the same tired old things that someone from Boston or NY would complain about. It got so bad I wrote down all their complaints on an index card and just handed it over when they started with their monoculture rants. It doesn't matter which nation in the west you are from, the elites form one gray goop of groupthink.
I do recommend travel, but go into the countryside or talk to people with different life values from your own, not just different stamps on their passport. Talk to a mormon. Talk to a devout Catholic. Talk to a rural minority from China. Talk to someone without a college degree from Spain. Go visit the a traditional village in Japan. Hang out with the cabbies in Holland who play chess all the time. There you will see diversity. Flying from Paris to London to Berlin and visiting hip coffee shops or tech seminars -- you might as well just stay in San Francisco and go out to a bar.
