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

It depends on location of Wi-Fi receiver. If it is inside of the DAC then USB and Wi-Fi should be equal since you’re not transferring music but data. USB might inject computer noise into your DAC while Wi-Fi can have dropouts. I had to change my Wi-Fi to 5GHz band because of that.

I use dual band router and dual band receiver (Airport Express).  My router has different IDs for different bands.  On my computer I select one corresponding to 5GHz (can be checked in router setup).  It also depends what do you use as receiver.  Many products, like Airport Express, don't allow direct connection (ad hock) and require router, while others might work direct.

What you deliver, by Ethernet, USB, Wi-Fi is just data. The question is how this data gets to DAC. My Wi-Fi receiver has analog and digital (Toslink) outputs. Digital could be better (280ps jitter), but fortunately my DAC has good jitter suppression, but analog (that I don’t use) is horrible (according to Stereophile) showing artifacts of around 1000ps jitter. If you cannot get data to DAC directly be sure that receiver/converter doesn’t add jitter. Many people go USB to USB/Spdif converter. Timing is not important with asynchronous USB but is important with Spdif (real time clock). If this timing is shaky you need reclocker.