Asynchronous USB vs. SPDIF with regard to DAC upgrade

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. 

@fastfreight yep. Highly dependent on the network cards implementation. As well as the streamer you’re comparing it to. Grimm MU1 is supposed to be a really amazing streamer/Roon core. 

Meitner EMM Labs, Playback Designs and dCS use Converse Digital streaming cards. They’re top notch. 
Bricasti uses some other network card…not sure what it is. 
When I had Lumin U1 Mini and Bricasti M3 I couldn’t tell the difference between using Lumin and built in M3 network card. Aurender N200 destroyed the M3 network card in sound quality. The difference was exactly what you described - the sound was more alive, more articulate and engaging. 
With Meitner MA3i I found the in-built streamer to sound livelier that the N200. But just as refined. It was a bitter sweet moment selling the Aurender as I really liked it. But I didn’t see any value in keeping it as it would be collecting dust. 
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My Innuos Statement has separate OCXO clocks for its USB and Ethernet outputs, and the USB is supposed to be very good.  I prefer the Ethernet output.

My Lampizator Goldengate3 has the JL Sounds USB upgrade card, I prefer the S/PDIF input.

I can run straight USB from Statement to GG3 with a FTA Sinope cable, Ethernet from the Statement to GG3 USB or USB from the Statement to GG3 S/PDIF.  I always prefer eliminating the USB output and or input. I do have USB in the chain, Sonore Signature SE Deluxe to Antipodes S20 but the net effect is beneficial.

I understand there are now USB outputs and inputs that sound better than S/PDIF AES/ABU but they are in very high end devices, or I have chosen my gear poorly.

Quality of a USB cable is very important. If you’re doing any comparison, use same level cables. 

A few thoughts based on recent experience. 

I upgraded from a Weiss 205 with Fidelizer linear PSU to a Weiss DAC 501 Mk II, and it was a significant upgrade. Other DACs I have owned are the Chord DAVE/M Scaler, Nagra Tube DAC, Ayre QX5 Twenty, Mytek Manhattan II amongst others. They are all good DACs, but I rate the 501 Mk II more highly than any of these, based on my personal preferences. I do not agree with the comment above that it is too detail oriented and analytical. It is neutral and accurate without artificial warmth or bloom, but is full sounding, as well as being musical and engaging. I have not heard the Berkely so don't know how it would compare. 

I have compared USB vs SPDIF and AES/EBU with the Weiss 501 Mk II a number of times, most recently in the last few days. I use a custom made PC based on a fanless NUC with linear PSU as a music server, and use its USB output either directly to the DAC, or to a Matrix X Spdif 3 and then SPDIF or AES/EBU into the DAC. The Matrix is also powered separately by a Fidelizer PSU. 

The USB input of the Weiss 501 Mk II is very good. But it definitely sounds better through the SPDIF or AES/EBU inputs (depending on the cable) via the Matrix X Spdif 3, rather than using USB directly into the DAC. The sound is richer, fuller, slightly warmer, and slightly quieter. 

Obviously, this is based on my specific configuration, and digital audio is very system dependent. 

The ethernet input of the DAC is also excellent, but again the SPDIF and AES inputs are noticeably better in my system. 

In other words, if you get the Weiss 501 Mk II - which you should certainly try to hear, because it is one of the best DACs currently available - you will need to experiment to see what input sounds best, because it is highly system (and cable) dependent. If you are using an external streamer, it will also depend on the streamer quality, including its own clock and internal PSU.