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

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.

@dougmint,
I became aware of the Bluesound at Audioconnection. Johnny switched input between the Bluesound integrated DAC and the Codex. 
It was significantly better using the Codex.
Which is not to say the Bluesound integrated DAC is bad. It just wasn't as 'alive' as the Codex.
So, if you want the connectivity of the Node, then by all means use it. But, if you want to upgrade the sound output, I would opt for an Ayre Codex, or a Schiit Gungnir, or Yggy. They can be had used for less than $2K and have the option of future updates (esp. Schiit).
Bob