What sounds better? A standard RP4 or 5 with USB or a RP4 or 5 with an additional hat and SPDIF?
Are you trying to start a holy war on this beautiful Sunday afternoon? 
Answer: it depends. Personally I think that the USB interface can be superior: it supports higher bitrates, the connection is bidirectional, and the DAC uses it’s own clock.
However, it relies on the USB implementation of the USB DAC. And from experience: that’s not a ‘walk in the park’. There are quite a lot of DAC’s that ‘get it right’, but I’ve seen the opposite of that as well.
If you’re not interested in bitrates above 96k, SPDIF is fine as well. And the galvanic isolation is a big plus.
To summarize: just try it out , and find out what for you is the best solution.
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Thanks! 
Well, I have Ropieee running an Allo DigiONE, and primarily use USB. However, with blind listening tests, I could not discern any difference between USB and S/PDIF.
Indeed, there should be no difference, even if jitter in the hat was moderately poor.
I originally bought the hat because I used to have a DAC that was susceptible to announing occasional clicks and pops (missed frames) caused by the Raspberry Pi 3s poor USB implementation (no issue with a Raspberry Pi 4 or newer.)
I can reach the dizzy heights of 192 kHz with the DigiONE, although most music I play is Red Book.
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I’ve got several systems and they use either USB, S/PDIF or Coax. I can’t tell the difference. Try it for yourself, but if it’s me I use what’s easiest (USB).
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The ALLO DIGIONE is a good one with very low Jitter, I use that one to supply several DACs via an active splitter. No difference to USB I can hear. USB just allows higher bitrates, but sound-wise no difference I can hear. The DIGIONE is as good as my HOLO AUDIO RED
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