Which approach is best suited for a migration to SAP S/4HANA and which path companies choose depends on numerous factors.
Technically, both migration scenarios are usually technically possible, as well as corresponding middle ways such as Bluefield (selective migration), which are usually based on system conversion and are characterized by selective data migration.
The decisive factor is the current stage of development of a company, what its current system landscape looks like, what IT architecture it is aiming for and what effort it wants to invest.
Basically, it can be said that the more complex, older and heterogeneous an ERP system is, the more likely it is that this system will hinder a company in its further development - and the more likely it is to consider the greenfield approach to migration.
On the other hand, the closer a company's system is to the SAP standard and the leaner its technical setup, the more likely the brownfield approach can be the right solution.