RME preamps have a transparent sound (“no sound”) and this is a good thing so they doesn't alter the original audio signal.
Because of this, they might sound quite equal. Simply high quality A/D conversion, not more.

If you like, you can take a look at the manual and read the specs. SNR and THD may not be much different.

The more recent / expensive models will most likely have better values, but this might not make a difference in sound. I would see this more as a requirement in studios where you record in high quality and high dynamic range, where you want to keep the noise floor as low as possible. But do YOU have such hard requirements?

If you have no hard requirements for that, then I would concentrate more on the features:
Number of ports / port density, Cost per port, Port reserves, ...

Do you work with dynamic mics, do you have low signals? Maybe silent recording? Then you might want preamps with higher gain range. In this case, have a look at UCX II, UFX II, UFX+, 12Mic, they have a gain range of 75 dB.

What could also be of interest: max input level, Autoset feature. PAD might not be needed if max input level is high enough so that the inputs have enough headroom for stronger signals.