If either are available, it doesn't matter.
S-Bahn is suburban train system which only goes below ground in the city center. U-Bahn is underground subway train system which covers the city center.
From the airport, you take either the S1 or S8 directly to the central train station.
If you are there for multiple days, (and assuming no 9 euro monthly pass available at that time), you can get a multiple day pass that includes that trip. There are machines, but can be confusing for that option. There is a desk where you can buy those tickets from a real live person near the S bahn departure.

A decent explanation is here: https://europeforvisitors.com/munich...ahn-s-bahn.htm