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?).

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.

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?

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.

Bluetooth is fine for a boombox, but not for serious audio. You may want to consider a Chromecast Audio. The default is a wifi connection, but it also has an optional ethernet connection if your signal is weak and you suffer dropouts. The internal DAC in the Chromecast is pretty decent, but it also has an optical digital output.

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.