Which device makes for a better digital music source? Computer or streamer?

I have tried a number of PCs, both custom built using standard components and "audiophile" PCs like the Antipodes. I have tried low powered PCs and high powered PCs. I have used linear PSUs and standard switch mode PSUs (but good quality, such as Seasonic). I have used audiophile USB cards and standard USB outputs, along with audiophile USB cables, and a variety of software such as Roon, JRiver, LMS and others. I have also used quite a number of streamers - SOTM, Lumin, Ultrarendu and OptricalRendu, currently Bricasti M5 (and others).

My conclusions are:

• A decent streamer always sounds better than any PC. This is true regardless of whether it is a NUC powered by a linear PSU, a high end gaming PC, or an audiophile server such as an Antipodes. Adding an audiophile USB card is a marginal improvement, but not enough to make it as good as a streamer.
• If you must use a PC as a source, a high powered PC (eg with an i7 or i9 CPU) with a standard switch mode power supply sounds better than a low powered PC (eg a NUC) powered by a linear PSU. Most "audiophile" servers are just low powered PCs with linear PSUs and a few tweaks.
• USB is not a good means of transmitting audio, despite its theoretical advantages. I  have consistently found that AES or SPDIF from a streamer sound consistently better than USB from either a streamer or PC.

These are the conclusions I have drawn from my listening experience over years. I am not suggesting these are absolute truths, and others may have different experiences. Other equipment may also produce different results. But these are the consistent conclusions I have drawn from my own experience.

Very interesting! thanks rossb....

The definitive answer without question is:  it depends.   Like most things the devil is in the details and generalizations do not carry much weight. 

Stands to reason streamer generally going to be superior to most server/streamer combo and far superior to general purpose computer. Noise is the enemy, segregating streamer from server or computer results in least amount of noise just prior to dac, certainly one of the most important links in network chain.

I really prefer no rendering taking place within servers, in this way they become simple network devices, minimize signature on sound quality.

Streamers are computers. So is your iPad and phone. Using a computer that's designed for streaming can be better than a laptop or desktop but there's no hard and fast rule. I'm using a NUC running ROCK if I'm going through Roon. I can also connect my speakers to my network and stream direct to them. I use a tablet as the controller.