Duet will be just plain stereo and better quality. Not just because it's an external device, so away from EMI; also it looks like it's not a cheap interface.
Strix Soar will have positional surround sound for gaming. Asus have been pretty lame in recent years, by not updating their drivers often enough. Some cards didn't get a driver update for 4 years.
To give Asus some credit though, after a quick look on their site, it seems there are newer drivers than I expected.