USB vs Coax Is one better than the other?

Rather than pinning all of the differences only on the USB cable vs the SPDIF one, which are passive components, keep in mind that we also have two substantially different active circuits involved, as antigrunge2 has already noted.

I'd give it good odds that the more natural sound you're hearing from coax may in your case be due to nothing more than it is a better circuit and more suited to quality audio than your USB option. (And, keep in mind, there is sending circuitry at the Aries end and receiving circuitry at the RME end, so even there the "blame" could be only in the USB circuitry at one end of the chain, or alternatively, the interaction of both ends.)

However, to me it doesn't seem to be a hard decision to go with the better sounding one.

Personally, I prefer USB for reasons regarding DSD and PCM resolution stated above. I think the EMI and galvanic isolation issues are solvable and have been addressed by multiple manufacturers. I think this is the way to go at least for now and the foreseeable future. 

I'm going through the exact same dilemma with a Auralic G1 streamer to Mytek Manhattan II DAC. The coax brings the vocals front and center with satisfying warm tones, but it lacks the detail and refinement of USB. I'm stuck deciding between the two because they both offer benefits! Nonetheless, my research has suggested coax vs. usb is component dependent. I believe coax relies on the streamer's clock while usb uses the dac's clock. Also, manufacturers implement usb and coax differently so there is no universal "superior" connection. Awaiting upgraded USB cable to match the coax cable tier for A/B comparison.