For transitioning to S/4HANA there are two fundamental options. You could either implement S/4HANA from scratch and migrate your existing data in a greenfield approach, or alternatively you could transition your existing ECC to S/4HANA including all of its customizations and data in a brownfield approach.
To make this decision, in my experience, the following dimensions have to be evaluated:
Data Quality / Unicode / Archive
When transitioning to S/4HANA, data matters big time.
To start with, S/4HANA does only support Unicode. If your ECC system was on multiple code pages (MDMP) a Unicode conversion would have to be included into the transition. Technically, the SAP S/4HANA transition tools (SUM DMO option) support this conversion, but more importantly, your business would have to provide the respective vocabularies, which could be integrated into a data migration approach.
While the reorganization and column store compression of an S/4HANA transition regularly shrinks your data footprint considerably, it remains important that you only take relevant data in-memory due to the huge price difference between memory and disk capacity. Therefore, you would either have to implement a data retention strategy upfront a technical transition to S/4HANA or integrate it into your data migration approach.
With S/4HANA, operational reporting could be run real-time on your transactional data without the need to load it into a data warehouse environment. However this implies, that there is no transformation step as it was in a traditional ETL approach any longer, so that the data quality has to be given in the system itself. To achieve this, there are two steps required.
Passive Data Governance
Either prior to your transition to S/4HANA or as part of the data migration, your data foundation has to be cleansed. Whichever you find easier or is more feasible will drive your decision.
Active Data Governance
Once cleansed, your data foundation has to be maintained clean, regardless a greenfield or brownfield approach.
Integration / Interfaces
In a real-time environment like S/4HANA, integration makes a bigger difference than ever, since real-time information on batched data has no benefit. Therefore any interface into S/4HANA or out of it would either have to be revisited prior to the transition or re-implemented with real-time in mind as part of the migration.
Versions / Patches
Simple Finance, on-premise edition 1503
Technically Simple Finance, on-premise edition 1503, is based on EhP 7 for ERP 6.0, which runs on SAP NetWeaver 7.40 and HANA as the database. Therefore a transition from an earlier release is very similar to a respective upgrade and a data migration less likely the preferred alternative.
S/4HANA, on-premise edition 1511
Technically S/4HANA, on-premise edition 1511, is based on S/4HANA core components that run on SAP NetWeaver 7.50 and SAP HANA as the database database. Therefore a lot of components have started to be changed and a transition would have more the character of a migration.
Modifications / Enhancements
Your modifications and enhancements would have to be checked for S/4HANA compliance. SAP provide a tool for this, that I described in my earlier blog: Make the S/4HANA Custom Code Analyser work for you and some of the corrective measures might be automatable, but there remains effort. Therefore, if you had a lot of custom code that is of no value any longer, a migration and S/4HANA compliant re-implementation of the still required enhancements might be an opportunity for further simplification.
Configuration / Features
Please be aware, that S/4HANA is work in progress and that not all the functionality that had been available in ECC 6.0 is available in S/4HANA, on-premise edition 1511 yet. Please check the User Assistance for SAP S/4HANA, on-premise edition 1511 for what functionality has already been made available. Also not all industry solutions and add-ons, whether from SAP or 3rd parties, are S/4HANA compliant yet. For SAP products the SAP HANA readiness can be checked in the PAM. For 3rd party software you would however contact the respective vendor. Missing functionality, a non S/4HANA compliant industry solution or add-on would of course prevent you from a transition whereas you might still be able migrate, if you could provide the respective functionality in another way.
User Experience / Mobility
The strategic S/4HANA user interface is Fiori, but in SAP S/4HANA, on-premise edition 1511 quite a few transactions are still rendered in SAPGUI for HTML leveraging Screen Personas. Therefore, regardless the approach, I would recommend that you make yourself familiar with those UI technologies and get the technical foundation for Fiori in place.
If you had a streamlined system in terms of code, data and integration or could get to one without too much pain, a brownfield transition to S/4HANA would be the recommended option. However, if you had a lot of technical debt in your system, a greenfield migration would be the more feasible option that would reward you with a clean and simplified result.