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

Though I have no technical knowledge, I have found USB to be the weakest link for music-like you say jitter and clock issues.
Ethernet, for me gives the best results, with WIFI (and having a strong signal) a close second.
B

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

@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

@OP,
I know it is a bit more money. In any case, the BS Node 2 has a pretty good DAC. I only mention the Codex as an upgrade option. (And, at $1300, used, it is a very good option).
B