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

Not much chance they will sound different. But mind you, you are not only changing the connection variable (cable vs wifi) but perhaps also the protocol (usb vs optical/coax?).

Willemj:  Can you explain more?  If you change between cable for USB and wifi, where does optical/coax come into the picture?

I am just not sure what conections you are really comparing. A (wired?) computer with a usb cable into a usb DAC I understand and should give excellent results. Unlike some believe computers are an excellent source, and usb to an external DAC is fine. But how does your alternative actually look like?

The 2 choices I am looking at are:

1) USB out of desktop computer into DAC
2) Wifi connection to DAC

I am wondering if there would be any difference in streaming Spotify sound quality between these 2 options, using something like a Bluesound Node 2, which can work with either option.

Same DAC? The DAC may have a very small sonic signature (but not much). If the rest is functioning properly, it is highly unlikelely there will be a sonic signature from that part of the chain.