It can't go 24 bit with coax? If it does, I'd get a sound card with coax instead.
I'm not familiar with any of this gear but I'm guessing its a non asycnch DAC and I'm willing to bet that the optical would sound better - or at least as good. I don't use usb at all and all my DACs have usb in. The optical ( in my gear) is prone to short bursts of static sometimes when switching sources or songs, esp if the sample rate is different but it sounds very good otherwise.
Also, if your DAC can use coax you can look into the hiface (usb to coax converter) as an alternative to changing your sound card.