Hi Javier,
In this scenario it does seem that SAP ERP Payroll would be a better solution than Employee Central Payroll, although it's not always as simple as that 
If you use Employee Central and Employee Central Payroll with Employee Central as the system of record then you would indeed have data in 3 places. In SAP ERP and Employee Central Payroll you would have mini-masters, but this data is replicated on a need basis so shouldn't pose any problems if your integration is well built.
The main thing is how Payroll is handled in Employee Central as there is mash-up functionality and display of payslip that is focused on Employee Central Payroll. The bottom line is that it probably depends on:
- What countries are in scope
- If Employee Central Payroll handles all of your required functionality
- Licensing
- Existing landscape
Best regards,
Luke