Comparing one S/PDIF vs another... both will probably work fine and sound great. Chances are the MAudio will not resample the data, while the on-board may resample everything, even if the sample rate doesn't change. They've improved things over the years, but most of the on-board and cheap-o consumer grade cards don't care about passing the exact same bits out, so long as the bits coming out seem to work. So if you've got the MAudio, I'd use it. For an affordable card, it's got a pretty good reputation.

- Jason