Here's a video comparison of VSS used by Creative (SBX) and Asus (DH)How is it compared to Xonar U7 MkII? Their prices are in the same ballpark but the U7 MkII is newer.
Yea I reckon a 50 ohm headphone with 116 dB SPL should be able to be driven with just the built in amp in a mobo, so I'm mainly looking for VSS/HRTF capability and clean ADC for the mic.
So just pick the one that sounds more natural. As for the ADC, I can't help You here although @Evshrug owns the Omni and could provide microphone samples if asked nicely.