Take a look at a latency comparison between different products, there is no significant difference with all RME products, be it FireWire, Thunderbolt, PCI/PCIe or USB.

https://www.tonstudio-forum.de/blog/ent … cts-en-de/

Over the years converter became much faster, but converter latency is still much lower compared to the latency for the transport between recording interface and the computer.

If you are monitoring a vocal recording, then you have a latency called RTL (round trip latency) which covers the complete signal path (Mic, AD conversion, transport to PC/DAW and back, DA conversion, monitoring).

Here the converter latency @44.1 kHz
UFX       AD 12 / DA 28 samples (0,27 / 0,63 ms)
UCX II   AD   5 / DA   6 samples (0,11 / 0,13 ms)

And now look into my blog article for the RTL and you will see that the (ASIO-) buffersize has significantly more impact on the RTL, even if the signal flow is only half of the RTL. This is the case when e.g. playing a virtual instrument where you have no AD conversion and no transport towards the computer. There you only have the processing latency in the DAW, transport to recording interface and D/A conversion for monitoring.

So .. if you compare the different recording interfaces you should better compare
- number and types of I/O ports
- special features (FX chip or not, Autoset, DURec, ...)
- stand-alone capabilities if needed
- to have a little reserve for future connection demands
- whether purchase of a used devices makes sense (higher age, the more likely it could fail)
- cross check, whether a MADI based solution would give you a better sustainability of purchase
- whether you want Room EQ and crossfeed (UCX II does support it, UFX not)
- ...