Raspberry board or USB DAC??

I’m a huge Volumio lover and great raspberry fan.
Willing to improve my sento system I am wondering:

Should I get a raspberry DAC board or get a good sounding USB DAC that I can connect to raspberry but also to another PC?

Thanks for your help

If you have any model Raspberry Pi up to and including the 3B+, then an I2S DAC hat is the best way to get good line level audio out.
All of those earlier models had poor USB implementation from an audio standpoint.
From the Raspberry Pi 4 it is possible to get good line level audio by using a USB connected DAC.

I have two DACs: an Allo Boss, and a Khadas Tone Board, each with its own RPi 3b+. Both sound pretty good to me. They are different, though.

It also depends on what you are streaming.
With a Pi 3b I had good results using and external USB DAC when streaming up to 176/24.
With DSD I was starting to get occasional hiccups.
Those hiccups went away when upgrading to an Asus Tinkerboard, and I assume the same would have happened with a Pi 4.
Quality wise, and as an investment, a good external USB DAC is the way to go. Nowadays good stereo amplifiers also come with good sounding DACs with USB input.

I bought the Lusya or Sodial es9038q2m hat board from ebay. It has the highly regarded almost reference level DAC chip.
I dont have any similar dacs to compare it with but sound quality really is excellent.
I think you would have to spend really a lot of money to better it. Endless detail, well behaved. The best I had before was a £3k Linn Karik Numerik cd player. It doesn’t begin to come close to the quality. Maybe the Chinese have copied the Audiophonics or Khadas similar dacs and much cheaper.
You can add an LCD, remote and volume which is very nice and then you have filters to change the sound.