RTSI is designed primarily for easy (software configurable) synchronization and signal sharing between DAQ boards.
PFIs are signal inputs to a DAQ card for purposes such as timing and other signals. This is to allow external synchoronization and other signal I/O to non RTSI systems.
There is no advantage of one over the other. If you want to share signals between two DAQ cards, you can either sit there and manually connect PFI signals together and hope you get it right, or you can just do it in software and merely connect one single ribbon cable between your cards (or eliminate this step altogehter with SCXI/PXI I believe). If you need to bring in an external timing signal, or output a sync signal from a DAQ card, you can ONLY use PFI.
