I have an FF800 and I am considering adding a UFX.
What are advantages / disadvantages /differences of UCX UFX features compared to each other?
For example:
UFX can record directly to USB stick and UCX will never have this feature.
Are A/D converters different?
Are preamps different?

What are differences in performance of UCX UFX compared to each other?

For example (my interpretation according to specifications posted on RME site, UCX spec listed first and UFX spec listed second):
UCX        UFX
AD, Line In 5-8, rear        AD, Line In 1-8, rear
Signal to Noise ratio (SNR): 111 dB RMS unweighted, 114 dBA     is better than     Signal to Noise ratio (SNR): 110 dB RMS unweighted, 113 dBA
Frequency response @ 44.1 kHz, -0.1 dB: 10 Hz - 20.6 kHz     is worse than     Frequency response @ 44.1 kHz, -0.1 dB: 5 Hz - 21.5 kHz
Frequency response @ 96 kHz, -0.5 dB: 5 Hz - 45.3 kHz     is worse than     Frequency response @ 96 kHz, -0.5 dB: 5 Hz - 45.5 kHz
Frequency response @ 192 kHz, -1 dB: -1 dB: 5 Hz - 90 kHz     is better than     Frequency response @ 192 kHz, -1 dB: 5 Hz - 66.5 kHz
THD: < -105 dB, < 0.0005 %     is worse than     THD: < -110 dB, < 0.00032 %
THD+N: < -98 dB, < 0.0012%     is worse than     THD+N: < -104 dB, < 0.00063 %
       
DA, Line Out 1-6, rear        DA, Line Out 3-8, rear
Dynamic range (DR): 111 dB, 114 dBA @ 44.1 kHz (unmuted)     is worse than     Dynamic range (DR): 115 dB RMS unweighted, 118 dBA
Frequency response @ 44.1 kHz, -0.1 dB: 5 Hz - 20.4 kHz     is worse than     Frequency response @ 44.1 kHz, -0.5 dB: 5 Hz - 22 kHz
Frequency response @ 96 kHz, -0.5 dB:5 Hz - 45 kHz     is better than     Frequency response @ 96 kHz, -0.5 dB: 5 Hz - 34 kHz
Frequency response @ 192 kHz, -1 dB: 5 Hz - 90 kHz     is better than     Frequency response @ 192 kHz, -1 dB: 5 Hz - 50 kHz
THD: -100 dB, < 0.001 %     is worse than     THD: -104 dB, < 0.00063 %
THD+N: -96 dB, < 0.0015 %     is worse than     THD+N: -100 dB, < 0.001 %