Asus tinker board s or raspberry pi4

What is the best for use with Volumio and external USB DAC ?

Thank you

Because of its Spdif -Out , I use the Tinkerboard S to drive my digital Cross-over in my active Speakers. Be aware that there is no volume regulation .

RPI4/4GB works flawlessly for me driving a Topping D90 DAC via USB. With the RPI4 you should consider a FLIRC heatsink case in preference to the standard plastic case.
RPI4 has plenty of CPU power and so is also is capable of running bruteFIR under Volumio for room equalisation if you wish to investigate that later as an upgrade. Not sure of ASUS in this regard. Ultimately, I’d look at the platform where you think most support and development effort is likely to be deployed - I not sure which wins here though!

I have both Pi4 and TBS. I ended up using the TBS due to the speed of built in eMMC which reduce boot up time quite a bit. Most of USB DAC doesn’t require a USB 3 speed (which Pi4 has advantage over TBS in this area ONLY if you connect a USB 3 device to it) so you should be fine with TBS. My DAC is Topping D90MQA and I notice the sound from TBS is a little bit juicier.

The other area that TBS is better in my situation is the 15.4" screen that I connect to the TBS via HDMI does go to sleep after 2 minutes of no activity (doesn’t go to sleep on the Pi4).

Here is what Volumio says about TBS over Pi4.

Here is my set up.

The video is probably talking about the earlier RPI3 rather than the RPI4 since the RPI4 has a dedicated USB chip and 1Gb Ethernet plus improved CPU.

I would wager that the Topping D90 would measure identically when fed with either the ASUS or RPI4 streamer and that a true double blind audition would reveal no differences either. When measurements extend to below the limit of human audibility, which they can quite easily with tools like the Audio Precision APX555, then listening tests will not be able to resolve any better.