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

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 

@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

willemj: I hooked up a Chromecast audio puck directly to my receiver via the analog output, and was not impressed with the sound of spotify.  Compared to a CD, it sounded really bad.  I would not consider it a good DAC.

gdnrbob: The Ayre Codex is much more expensive than the Node 2, so out of my price range.  But I did compare the Node 2 ($500) with a Linn DAC ($4000) and didn't really notice any difference, so I am thinking the Node 2 must be pretty good.

@dougmint,
I became aware of the Bluesound at Audioconnection. Johnny switched input between the Bluesound integrated DAC and the Codex. 
It was significantly better using the Codex.
Which is not to say the Bluesound integrated DAC is bad. It just wasn't as 'alive' as the Codex.
So, if you want the connectivity of the Node, then by all means use it. But, if you want to upgrade the sound output, I would opt for an Ayre Codex, or a Schiit Gungnir, or Yggy. They can be had used for less than $2K and have the option of future updates (esp. Schiit).
Bob

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.

willemj:  Why are you bringing up Bluetooth?  There has not been any talk about Bluetooth in this thread.  We've been only talking about wifi, ethernet, and USB.

Also, if you read my earlier post, I did try a Chromecast audio puck into my receiver, and was not impressed with the sound quality at all.

gdnrbob: I was planning to use the Bluesound Node 2 as a streamer and a DAC, as it does both.  Why did you add an Ayre Codex into your pipeline?  Where you not happy with the sound quality coming out of the Node 2?

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.

Bluesound is excellent. /customer service is top notch, too.
I bought my Node's used, and Bluesound gives fast support when I had a problem with my players.
I bought an Ayre Codex to serve as DAC, the Node's are just for networking.
The 2 make a great combo.
B

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.

gasbose.  I am still a newbee, but I believe I would be correct to say that you cannot expect great performance from a $25 DAC from Amazon.  I just tried a Google Chromecast audio "puck" ($35) into my receiver via the analog output, and I was not impressed at all.

I just bought a used Cambridge Audio 2 channel amp, and am thinking of getting a Bluesound Node 2 for the streaming and DAC functions.  I listened to one at a store, and it was decent.  I also listened to a $4K Linn DAC, and at least for Spotify, could not really hear much difference when compared to the $500 Node 2.  I am going to try using some old Klipsch KG4 speakers I've had in storage for 10+ years.

Thanks, Dougmint for raising this question; I have the same one.

I've recently been experimenting with Tidal, and am connecting to my preamp via a fairly generic $25 bluetooth 'adapter' from Amazon. I'm guessing this contains a wifi receiver plus a DAC. The sound, while not bad is a bit plasticky, like Muzak.

I've been wondering if the weak link is the DAC or the fact that it's bluetooth (as opposed to a hard-wired connection). Could I improve the sound by getting a DAC that accepts a USB? or Coax? Or would it just depend on the DAC?

Gasbose

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.

kijanki.  Great information.  Thank you.  Is there an easy way to tell what band my wifi is operating on?

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.

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. 

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.

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.

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.

willemj: Did you read my original post?  Yes, same DAC.  My question is written:

If the DAC, amp, speakers are the same, will one of these 2 options likely provide better sound quality?

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.

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.

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?

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

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?).

Though I have no technical knowledge, I have found USB to be the weakest link for music-like you say jitter and clock issues.
Ethernet, for me gives the best results, with WIFI (and having a strong signal) a close second.
B

I have heard that computers are noisy, and that their internal clocks can cause issues with music playback.  I was thinking that this would impact the playback through the USB cable.  Do you get the same jitter/clock issues regardless of if you go through a USB or wifi?  What about if I go through an ethernet cable from my wifi hub into the DAC?  Any difference to be expected?

Nope, they'll sound the same.