Can't say which is most preferred, everyone seems to have their own preferences and there are(were) quite a few vocal supporters of other opamps. Honestly I haven't read the m-stage thread in years, and that thing is like 900 pages long.
But from memory the 627AP was one of the early opamps that was tested with the m-stage and most people seemed to see it as an improvement over the default one.
USB version adds 30 bucks to the price and is probably only useful for people who don't have a DAC/don't want to use their onboard mobo sound.
Edited by 0o0o0o - 1/30/13 at 11:33pm