Both cards are basically identical. Aside from the slot difference (PCI versus PCI-E), there are two key differences:
1: The STX requires an extra power connector
2: The ST adds a jitter clock, which ever so slightly increases overall quality.
The differences between the two are negligable, so use whichever one best fits your motherboard. I believe both can be connected to the H6 card to allow full multichannel hookups, though if you need that, the Xonar Xense might be a better option for you.