The best USB connection vs high quality soundcard

I am very interested in how these two compare i.e. the usb output using a high quality usb converter(please state which one) vs the digital out from a very high quality soundcard such as a Lynx L22 or a topline RME.
What are the sonic differences and which did you end up preferring.
Never used a card, but very much like the Waveterminal U24 as a USB convertor. No longer available except used and reamaindered.

I have used a Metric Halo ULN 2 interface run through a Z systems RDP-1 and then to a Audio Aero Capitole DAC as well as running a Mac mini hooked via USB to a Wavelength Cosecant USB DAC. I prefer the Wavelength DAC, to me it sounds better and is simpler to use. The convenience of using itunes to store and catalog all my music has made CD players obsolete for me.
I'd also second Ejlif's knod to a USB DAC. That's the direction I've taken as well, but I still use the U24 occasionally in my second system. Since you were asking about an interface (convertor) I addressed that, but the DAC is a better way to go. I use a Waveterminal Brick and love it. It is the baby brother of the more expensive Cosecant Ejilif uses. If you can swing the extra $ go for the Cosecant, but if not the Brick is a great USB DAC. Unfortunately either one are very difficult to find on the used market, but do show up occasionally. Other choices include Apogee's Mini Dac and the Scott Nixon Chibi. There are others and will be more as PC Audio becomes more widely embraced.