The line-in connection would give you higher quality audio. But depending on the audio source, you may not notice much of a difference between the two.
Apart from the hard to detect sound quality issue, there is the higher music play stability afforded by a wired connection compared to any wireless one; wherever convenient to do so, go wired.
In any circumstances a cable connection is always better than a wireless connection.
I am a computer engineer and the number of times I go to people whose printers are next to their computers and they connect via wireless.
Where is the router? In another room. So the signal goes from the computer to the router, back again to the printer, back to the router and back to the printer.
Why not just cable it with a USB. More stable
In any circumstances a cable connection is always better than a wireless connection.
I have all my Sonos zones in a large open central apartment core, in line of sight of each other. Sonos music play was stable but when I had the chance to run ethernet wires there, I did so and all Sonos units now are ethernet wired back to the base station. I haven’t seen a signal drop of any kind or duration since I did that, and for the units that do not need them, their WiFi radios are turned off now.