Everything else being equal, should I expect better Spotify quality through wifi or USB?

What you deliver, by Ethernet, USB, Wi-Fi is just data. The question is how this data gets to DAC. My Wi-Fi receiver has analog and digital (Toslink) outputs. Digital could be better (280ps jitter), but fortunately my DAC has good jitter suppression, but analog (that I don’t use) is horrible (according to Stereophile) showing artifacts of around 1000ps jitter. If you cannot get data to DAC directly be sure that receiver/converter doesn’t add jitter. Many people go USB to USB/Spdif converter. Timing is not important with asynchronous USB but is important with Spdif (real time clock). If this timing is shaky you need reclocker.

Bluesound is excellent. /customer service is top notch, too.
I bought my Node's used, and Bluesound gives fast support when I had a problem with my players.
I bought an Ayre Codex to serve as DAC, the Node's are just for networking.
The 2 make a great combo.
B

Bluetooth is fine for a boombox, but not for serious audio. You may want to consider a Chromecast Audio. The default is a wifi connection, but it also has an optional ethernet connection if your signal is weak and you suffer dropouts. The internal DAC in the Chromecast is pretty decent, but it also has an optical digital output.

gdnrbob: I was planning to use the Bluesound Node 2 as a streamer and a DAC, as it does both.  Why did you add an Ayre Codex into your pipeline?  Where you not happy with the sound quality coming out of the Node 2?

willemj:  Why are you bringing up Bluetooth?  There has not been any talk about Bluetooth in this thread.  We've been only talking about wifi, ethernet, and USB.

Also, if you read my earlier post, I did try a Chromecast audio puck into my receiver, and was not impressed with the sound quality at all.