Unless the onboard has a high noise floor (higher than the noise floor of the source) or other discernible imperfections, I'd go with the onboard.

The Onboard drawbacks are that the can have a higher noise floor, since they are close to noisy digital stuff, and they do have a higher CPU usage (though that normally concerns gamers).

Unless there is a problem with the onboard, a normal priced soundcard might not be that much of an improvement - bizarrely, at the PCAVtech site, a generic Crystal CS4232 chipset card gets some really excellent results - up with cards that probably cost ten times as much