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

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

willemj: I hooked up a Chromecast audio puck directly to my receiver via the analog output, and was not impressed with the sound of spotify.  Compared to a CD, it sounded really bad.  I would not consider it a good DAC.

gdnrbob: The Ayre Codex is much more expensive than the Node 2, so out of my price range.  But I did compare the Node 2 ($500) with a Linn DAC ($4000) and didn't really notice any difference, so I am thinking the Node 2 must be pretty good.

@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