Hi Hubery,
To send the data from SAP ECC, I would suggest to use IDoc or proxies, based on the availability of standard options for triggering changed record. Or if it is a full extract of data from SAP system, you can write a scheduled report from SAP ECC, which can send out all records from SAP system.
For the receiver side, check with the 3rd party system, to see what kind of interfacing options they support (SOAP, REST, File etc.). Note that IDoc support only async mechanism, so if you want to integrate using that that and the 3rd party only supports synchronous mechanism, you will need to use async-sync bridge modules in receiver channel. Proxy on the other hand supports both async and sync mechanisms.
I would not recommend using RFC, as you will loose on monitoring and error handling capabilities.
Regards,
Sanjeev.