ERA 300 Combo adapter

It would be equal, not better, than a WiFi connection. That’s the way Sonos is designed. 

Wifi, any version, and Ethernet are going to give equally high quality, they deliver identical bits.

The 3.5 adapter is going to have an analog/digital conversion added to the above. How much impact it has isn't documented. Better or worse than the Port or other Sonos analog input is again undocumented. 

Bluetooth, no clue, but based on other devices (not Sonos) it always remains my last choice. Digging into the connection details for the exact quality connection it will deliver between your transmitting device and your specific Sonos product might provide some insignt as to signal to noise, bitrate, bit depth and overall quality.

If you used the dongle’s network connection, quality would be the same as WiFi. Using the 3.5mm input would add another A to D conversion to the signal path. I suppose that there could be a discussion about the quality of an A to D or D to A conversion somewhere else in the path to the 300. Each conversation risks some loss of quality.

Thanks for the responses. Found another review article advising that switching from wi-fi to Bluetooth reduced the noice floor and seemed to markedly improve sound quality. Author seemed unaware of the direct Ethernet option, but I’m assuming this would have the same effect…..

https://theaudiophileman.com/what-a-waist-sonos-era-300/

Stop reading nonsense like this.  Matter of fact, avoiding anything associated with the word “audiophile” is the best advice I can give. 

Bluetooth in no way could be superior to streaming using the app.  Bluetooth is lossy, streaming is lossless (depending on your source/provider).  Also, there is absolutely no difference using Ethernet compared to wireless.  Both deliver the source stream as is, there is no such thing as quality that varies according to connection type or reliability, the stream arrives as is no matter the connection.  If anything interferes with that, you don’t get a subtle drop in fidelity, you get very noticeable dropouts. 

Never good when I'm bored so I'll read that article.

Oh goodness! I need to order a whole pile of new power cables as the ones Sonos is shipping compromise the sound quality.

The rest of it is even worse.

Should’ve trusted my instincts and abandoned the article after the too-cool-for-school intentional misspelling of the word “waste”.

I made it half way…I think. When they started talking about power cabling, I stopped. This person is a bit unsophisticated. As ​@jgatie suggests, I should have just stopped with the uneducated use of the word waist for waste. I’m of the opinion it wasn’t ‘ironic’ or intentional.