Streamer output options

What DAC is it? There’s no definitive answer to your question. Highly dependent on dac and streamer. USB implementation in both units, quality of DAC clock and jitter rejection properties. If you use spdif you’ll be relying on the clock that’s in your streamer. 

I have a Mytek Liberty 2. It's a couple of years old, so totally obsolete! 😀 

No not really obsolete. With that DAC I would try both USB and spdif. Need warm sounding cable with mytek. 

I never use usb. Spdif has restrictions on what you can send thru it. If you are paying hires music, say dsd128 or dsd256, you are out of luck. Also, you are restricted to 192/24 files. 
If you are only using a cd player, this wouldn’t matter. I haven’t used a cd player in almost 20 years.

There are some very good low-priced I2S/SPDIF streamers that have extremely good specs, like the Holo Red and the Mercury V3.  I use the Mercury, which is only $500 and far surpasses the USB output of my Mac Mini server in terms of accuracy and cleanliness.  But I don't do extreme upsampling or DSD.

OTOH a friend of mine (who has what I consider a reference system) prefers USB and has a custom-built streamer with JCat USB and network cards.  If he weren't happy with it I would hear about it. ;-)

I think in the end it depends on what level of playback you want and if the higher resolution options are worth it to you.  It seems like you need to spend a bit more to get high-quality USB output, but that shouldn't stop you from going that route.  Relatively affordable USB streamers from SOtM and Sonore are very popular and have good reputations.  Then there's Lumin, Aurender and others that seem to be very satisfactory in terms of USB quality.  At that point, you need to look at the connection interfaces, which vary in ease and quality.  There are a number of threads here about that.