Which do you find gives lower latency and CPU usage?

I use USB, make sure that the RME is the only device on that hub (I think that's the right term).
IIRC, RME states that USB uses 32 fewer "hidden" buffers; BUT, performance is system dependent.
I've never tried the UFX with firewire, but my FW connects via firewire; so that's not an option.
 
HTH,
Tom

Usb

I use USB as well.

RME UCX user here via USB.
 
Don't use Firewire, but I recall seeing some performance data on an independant test site for the UCX comparing Firewire and USB. There was no difference to speak of, so that made USB a no-brainer for simplicity sake.