rca or usb digital

You will want RCA digital cable, 75 Ohm impedance.  USB can work, although it is best with galvanic isolation and jitter reduction.

Depending on what you are streaming from or using as a server. With my Innuos Statement I use USB to my 680D DAC. No jitter, noise or issues. The only way to connect. 

i have a surrender n20 coming

With an Aurender you are likely to get best performance from the AES connector. This is what I found best from my W20SE. 

Same question, three different answers...that's what i was afraid of. ugh

Whatever sounds better and what the manufacturer of your DAC recommended.

I have a LessLoss R2R with the outboard 640X firewall and 3 inboard. Also use a Innuos Zenith with the Sense software. Zafino Cable USB. No clock as there is no need for it. I also do not use MQA as it a bet of a joke in my opinion. Use Quobuz for streaming and mostly radio..Pardise. .1FM.

Sounds spectacular in my system. Lower grade stuff will require more insularly items to make your digital sound better. DDC, Clocks, ….just my options so take it with a grande and use what sounds best to you. 

How do you like the N20?    I have the N200 and am blown away at how great digital can sound .    I Should have put money in the digital front end a long time ago....   just as well because I was waiting for streamers to mature a little.   

@ckr1969 With the Aurender N20, AES is best, followed by RCA / Coax. This is due to the N20’s clock being far superior to that of the one inside the Aavik. Given the way each cable’s interface works, AES and Coax will use the clock in the Aurender. USB uses the Aavik’s clock. Given the Aavik has no AES input, I suggest you go with a digital RCA / Coax. 
 

@oddiofyl The N200 is the sweet spot in the Aurender lineup in terms of price to performance ratio. At 2x the price, the N20 is 20-30% better, and it excels more in tonal density and soundstage size. Clarity/articulation is quite good already at the N200, and while the N20 improves slightly, the benefit is more in the tonal density, dynamics/punchiness, and more natural delivery, whereas the N200 is a bit thinner sounding in comparison. I just performed this comparison less than two weeks ago.  The difference in the N20 and N30SA is even smaller, but it is that last inch that makes the difference, despite again double the cost. The N20 is punchier than the N30SA, but the N30SA is more effortless and refined, delivering more of everything in a more natural presentation. 

I think you need to try both and see how it sounds in your system.  Asking us and you're likely not going to get the answer you want.   I just completed 75 hours of A/B testing between RCA, USB, and S/PDIF cables.  It was worth the experiment.  The key is to find a dealer who is willing to let you demo various cables.  Gestalt Audio in Nashville ( Colin King) allowed me to do that and I live in Arizona.  

There is no apparent advantage in using USB because it's a PCM only DAC.

You will be able to play DSD as DOP using the N20s settings but not native DSD.

Upsampling via the N20 is available for AES/EBU and COAX outputs.

I use a Musetec 005 standalone DAC with my N20, I highly recommend it, you will then be able to play native DSD via USB.