> How do you pick up one over the other?

I believe you do not have a choice. iOS, Android, macOS, Linux Desktop goes to Mono. No choice there. Application Servers go to .NET Core. No choice there considering performance and Cloud deployability. Windows Legacy goes to the .NET Framework. No choice there. Modern Windows App goes to UWP (another .NET Core).

I do not see a choice except when you want to run a service on Mono. Which everyone will tell you not to do.