FTP best practice with XI

What is the best practice in using FTP adapter which is in vendor network ( external) ? If the connection parameter is set as permanent, or file per transfer, whats the major difference in performance and security? The vendor FTP has uname and pwd for each interfaces from SAP system. If I use file per transfer, everytime the data is sent, FTP takes it in a directory using uname and pwd access. Will it affect the performance ?

Thanks

Ricky