What's the difference between installing a "Default Instance" vs a "Named Instance"?

You can install only one default instance but can install multiple named instances.

The underlying difference between a default and named instance is mostly a matter of network connectivity. Clients can connect to the default instance using only the host name over the well-known 1433 port. To connect to a named instance, clients specify the host and instance name (e.g. "MyHost\My_Instance").

If you're only going to install one version of SQL Server on this machine, go with a default. If you're planning on multiple instances on this machine, it's up to you whether you want one named and one default, or both named.