Go with the Asus Xonar Essence STX. The sound quality difference between the ST and the STX is not much; it's just that the ST has the ability to add on another component for surround sound (separately purchased).
I would go with the STX; PCI will probably be around for quite some more time, however I feel that PCI-e is a better investment. Better to be safe than sorry 
P.S. I have the Asus Xonar Essence STX PCI-e + Sennheiser HD 598's, and I absolutely love them. They complement each other very well. I'm still trying to learn about more tweaks on my soundcard, but as of now, it's a 10/10 in my book 