Since my setup is pretty much the same as yours - can you describe more in detail the perceived audio differences between Monitor 01 USD and M2tech converters?
I am, at the moment, using Monitor 01 USD (XP with latest 18.104.22.168 drivers, BNC output) connected through externally powered USB hub and I'm very pleased with it. But if this M2tech "dongle" is so much better...
So, it's really up to you. If you can sell musiland USD, I'd buy m2tech and do a honest comparison. Whichever you don't like, you can always sell it back. I chose m2tech mostly due to:
1. Easier communication
2. No need for extra USB Cable.
3. BNC Option
4. Two very stable oscillators for jitter management.
5. Forth-coming Mac Driver (10 days or so now).