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

@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

To make better Spotify audio quality to DAC, you can use Free Spotify Downloader for Mac to aujust Spotify quality by changing the bite rate to 320 kbps. And then you can convert them to USB flash drive and listen to Spotify music via USB from desktop computer to DAC. 

To get better quality, you can download the Spotify songs offline. Then you shold get rid of all the sound problem maybe caused by the internet issue. All you just is Spotify Playlist downloader for Mac. For more info, you can visit Here. 

Doug,
Honestly , Spotify is super convenient and nice for the car or casual listening, but audio quality is quite poor. If that's your source, you will never good quality sound. Your original question becomes a coin toss, because both will likely leave you ultimately dissatisfied.
However, the Bluesound suggestion is a good one, in that if you later move up to Tidal, Quobuz or better yet, a NAS streaming your own CD quality and high rez files, then you will likely be very happy.
With Spotify you are going to war with a pocketknife. It's gonna be hard to emerge victorious. Cheers,
Spencer