Asynchronous USB vs. SPDIF with regard to DAC upgrade

It’s honestly a little complicated.  It’s true that USB Asynhcoronous is theoretically the best jitter performance, but it’s also true the Berkley Alpha is a very good sounding DAC.  I'm not sure how they deal with signals > 192kHz or 32 bits, and that may be the big limitation of S/PDIF, which USB 2.0 overcomes. 

DAC’s do very well with the "normal" jitter of a modern streamer.  The PLL circuit and FIFO buffer will adapt, and so long as the streamer’s clock doesn’t suffer from truly terrible jitter performance you should be OK. 

An alternative however may be the Bricasti. 

The overriding factor should be which DAC will best give you the sound characteristics and improvements you’re looking for. 

Your Lumin U2 Mini can be eliminated as I mentioned in your other thread. It’s a good streamer but it doesn’t beat network cards in Bricasti or Meitner. If you’re a Roon user, just get the highest quality DAC with built in network card. Examples - Bricasti M21, Meitner MA3i, Mola Mola Tambaqui.
I eliminated Aurender N200 with Audience Studio One USB cable, with Meitner MA3i. I compared the crap out of it with and without Aurender, using native conductor and using roon. I preferred the Meitner as Roon end point as it sounded better to me. Sold the N200. I can tell you that the N200 is noticeably better than Lumin U1 Mini and therefore it’s also better than the U2 Mini as I’ve not heard of U2 Mini being an upgrade over the original. 
Keep your options open. 

Makes sense @audphile1 .  But, I have direct experience with MM Tambaqui first using its internal Roon Bridge and then with the Auralic Aries G2.1 and 2.2. and Grimm MU1

The sound was more lifelike and palpable with the streamer.  The G2.1 was annoyingly unstable, the 2.2 rock solid.  But the Grimm upsamples everything to 192 and sounds great using AES.  Better than no upsampling and USB to my Tambaqui.  So the Grimm MU1 and the Tambaqui  are a great combo, using AES, and it just shows how equipment can be so specific.

First off, I think you'd be hard pressed to find any decent audiophile-oriented DAC out there where the USB input is not asynchronous these days. Anything older than maybe 2012-2014, then you'd have to double check the design specs, but almost everything beyond that will be async. 

Second, like many other things in this world, there's no single correct answer as to what is better. Async is not a magic bullet although it was a big improvement over the old methods in a variety of ways. Still, I have tried a number of great DACs that sound best from USB and others where USB is my least favorite. Sometimes AES is your best bet, sometimes I2S (but watch out for compatibility issues, there are many standards), and sometimes even plain old coaxial SPDIF. It really comes down to trial and error.