Which EFSNet option should I choose?
1) EFSNet - Legacy
2) EFSNet - Secure Transport Link
If you have not yet contacted First Data and setup your new clientline account then use this gateway option.
Secure Transport Link
After you have contacted you can change from the "Legacy" to the "Secure Transport Link". No other action will be required.
Note: it is a good idea to test your account with the Credit card test numbers after making these changes.