I´m currently working with a Fireface 400, but also had a Multiface I years ago.
Both work perfect, without any problems.
With the PCI/e version you could in theory even reduce the latency a little bit more, because Firewire always incorporates some ( very small) overhead.
But all this is pure theory. Both interfaces work out of the box.
The point here IMO is the quality of RME´s audio driver and firmware, which is top notch. I also had an EMU interface for some time. The audio quality was great, but the driver stability was totally rubbish, although it used the same hardware PCI port as the RME did.
90% I think is software & hardware implementation, whereas the theoretical bandwidth is much less important for audiointerfaces.
Don´t know how stable their newer USB solutions are, but with any of the other interfaces, either Firewire or PCI/e, you can´t go wrong.
P.S.: "High quality 'clock' ?? What is this?!"
If you like, you can implement an external "state of the art" masterclock instead of using the interface´s own digital clock. This can make sense if you have lots of digital equipment and want to glue any effects processor, audio interface, synth , digital mixing console etc to one "master beat". Usually these clocks offer extremely stable clock generators and operate very jitter-free. But they are expensive at the same time.
IMHO only interesting for professional applications in bigger studio environments, since even the internal clocks nowadays have improved a lot and offer good quality.