spend on the streamer or on the DAC ?

To me it can make sense to combine streamer, DAC, and preamp in a single box (then just add a power amp and speakers ... or active speakers).  That way you're paying for just one chassis and you reduce the number of interconnects.  This "integrated preamp" is a modern, digital alternative to the traditional integrated amp.  Many products are available that combine 2 or 3 of these functions. Prices range from ~$500 for Bluesound to nearly $40K for the T+A SDV 3100 HV.  

I have owned the BS Node 2i, the iFi Pro iDSD, an ultraRendu (streamer/NAA only, no DAC/preamp), and the Matrix Element X. Lately I've settled on the Matrix Element X, which has an excellent DAC, WiFi/ethernet streaming, decent input switching functions, and very good volume control. As far as I'm concerned, all these devices perform the ethernet streaming function about equally well (all else being equal, or nearly so).  As for the DAC per se, I'd find it challenging to isolate the contribution it alone makes. FWIW, in ASR testing, many devices (including the Element X) have measured better than the Node 2i.
https://www.audiosciencereview.com/forum/index.php?pages/Audio_DAC_Performance_Index/

In this setup my Node2 does the full MQA unfolding before sending it to the DAC (via digital coax).

Actually, no.  You only get the full unfold if you send output to the analog outs of the Node 2.

https://support1.bluesound.com/hc/en-us/community/posts/360037876633-MQA-Confusion

I have yet to see a technical reason as to why the streamer does or doesn't matter, but my experience was that an external DAC did improve sound quality (in my case an Auralic Vega), but that replacing the Node 2 with a higher end streamer/DAC (for me it was the TEAC NT-505) resulted in a greater improvement in sound quality, a less "digital" sound, and a larger more "holographic" soundstage.  As others have said, the app you use to control your streaming is also important.

One point that seems to be getting lost in the conversation is how your streamer is connected to the internet. I noticed a huge difference in all aspects of the sound after I ran a direct wired connection to my streamer. Wireless is convenient but igo with the direct connection when ever you can. I also added an IFi filter on the connection and it cleaned up the jitter and noise.

Thk u all for the useful insights. I  am attracted to a single-box solution (streamer/DAC, and I'd like to
keep my MOON amp,and MOON seems to make nice DAC/streamers BTW). In this regard there is the issue of obsolescence: where does technology move the fastest, on the streamer side or on the DAC side? This might be an issue if one considers future upgrades. Regards. Olivier

I am waiting for a Denafrips Pontus DAC to arrive and I will team that up with a Bluesound Node 2i streamer for the time being. Read the reviews on the Denafrips as they read to be an excellent performer. 

Bill from Nova Scotia