Best protocol suggestion

Hi everyone

The client will place xml files in a folder. Client insists that security is a big concern for them. Cient also wants the whole process to be automated. So they want us to go for either SFTP or AS2. Which option would be the best? Or is there any third option which offers better safety like SOAP or http adapters at sender side. Or any other option that can take care of security and automation better?

Harish