Hi Mike,
Both ways will work fine, however using the CL_GUI_FRONTEND_SERVICES will mean there must be a SAP GUI session for this to work, i.e. no background jobs. Also the Windows user running the SAP GUI session must have appropriate access to both folders.
Using the OPEN DATASET command will always work the same in foreground and background, but this time the Windows user SAP runs under (ask Basis team) will need access. As this is a lower level process implemented via system calls, this way is probably faster. If the folders are actually on different servers, zipping the file for sending (using CL_ABAP_ZIP) should help on the network transfer time (depending on the complexity of the file).
Cheers,
Phil