Which region should I choose?

Latency between different places on the planet is something that can't be beaten - unfortunately the speed of light isn't infinite. Hosting your application in the Seoul region would provide a great experience for users in South Korea but probably less so for users in Peru. Same goes for using the Sao Paulo region.

You could host in a region that is (roughly) equidistant (in terms of latency) from both locations - that way users would get (more or less) the same experience.

In this case I would advise you to test your application in various regions to determine which supplies the best user experience.

Check out this video for some region selection tips - https://www.youtube.com/watch?v=QJE7TOaemNY&t=4s

Key areas to keep in mind:

• Compliance - Do you have any regulatory or policy considerations where you need to store and process data within a specific country, etc.
• Are all the AWS services you need available in the region? - Not all regions have every AWS service and feature capabilities.
• Region proximity to your users - Testing is necessary here, but in general, the closer the AWS region to the majority of your users, you'll have lower latency and a better end-user experience
• Costs - AWS service and data transfer costs. Not all regions cost the same. Depending on what services you'll be running and the amount of data transfer, there may be tradeoff decisions to consider, like accepting higher latency for users to save costs by picking a region that's not the closest to your users, but may be less expensive to operate out of.

The video dives into these key points a bit deeper.

For your scenario, once you've considered the above aspects, you may want to think about a multi-region design to better serve customers around the globe. You can improve your customer experience greatly by having them handled by the closest AWS region, then you can also leverage these additional AWS regions as Disaster Recovery regions should you need them.