I don't really think that external sound cards present less compatibility problems than USB DAC's. Many manufacturers (including Creative, I don't know about M-Audio, to cite the ones you mention) doesn't provide drivers for Linux, MAC or both. Some don't even support Windows Vista yet. With an USB DAC you're more likely to have compatibility.
An example: the Firestone Audio FUBAR III is compatible with Windows XP (and should be with Vista unless M$ s**rewed anything), Linux and MacOS X as far as I know. Any PCM2702 based USB DAC will do the same.
Any external sound card using USB should perform the same, but don't count on drivers for Linux or Mac, so any "supacool" feature may be unusable then.
For highest-quality USB DAC's... I don't know. Depending on the chip the used and the things they added in the USB interface part, they may be unusable on Linux and Mac. Anyway, I think that it is easier (on the manufacturer part, I mean) to stick to universally supported chips and build the audio quality on the opamp part and other places.
If you're interested in an USB DAC, check the chip and that will give you an idea about compatibility. For an external soundcard, check if the manufacturer supports some OS other than WinXP. For Linux, check the ALSA project website, since although most manufacturers don't support free software, they don't cripple their hardware enough to make it unusable on free software
Good luck!
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