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

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.

See post by Gasbose. As for the Chromecast, I am personally very impressed using it into a pretty high end system with Quad electrostats. The optical output is bitperfect, and the internal DAC is really rather good for 16/44.1. These days, 16/44.1 is not a challenge for any DAC chip.