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

Sorry I overlooked the 'same DAC'. If I understand you now, this is a DAC with both a usb port and a streaming facility, or, to put it the other way around,  a streamer with its own DAC. I would be surprised if there are any sonic differences, so I would go for the most convenient option.

The short answer is it depends. The variables are the quality and length of your USB cable, the performance of your laptop, WiFi signal strength and internet speed, and the quality of your streamer to a lesser degree. A better comparison is between wired Ethernet connection and WiFi and even that would depend on the quality and length of your Ethernet cable vs. your WiFi signal strength and internet speed. The ultimate arbitrator is your ears.  Just keep in mind to unplug your laptop and run on battery if/when streaming from your computer since the AC noise is a major culprit in introducing noise in the signal chain.

kalali: thanks for the response.  I don't have a laptop though.  As I said in my original post, I have a desktop.  Having said that, since you brought it up, should I expect better sound quality through an ethernet cable to DAC or via a wifi stream.  I'll add that I have high speed internet, and the DAC and wireless router will be very close to each other.  If you tell me Ethernet should be better, then I won't look for a DAC that accepts wifi.  If wifi is expected to be better, then I won't look for a DAC with ethernet.  Or I guess I can look for one that accepts wifi, ethernet and usb, and try them all.  I was just thinking someone else may have already tried this and could provide guidance.

Given your set up, I don't think you'll hear a difference between ethernet and wifi and either should sound better than USB given the same quality content. 

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.