"the best" is very likely the upcoming zxr. far superior mic port, same (replaceable) opamps, sameish dac, same headphone amp, better or same snr depending on mode, with a native pcie interface (the stx uses a bridge chip). this causes less jitter and latency, which might be unnoticeable, but a product can't be called "best" when there's a similar product out there that's either the same or better in all regards. however if creative forces you to buy the DBPro (a niche "pro" recording daughterboard) with it, the £200 msrp for the package is high - anywhere from ~£150-£180 and below though (with or without DBPro) and it probably beats the st/stx in value (subject to reviews!)
the zx and zxr come with a headphone+mic extension on a remote that has a handy hardware volume control, so your headphones can be connected to something on your table. the zx loses out on internal quality, but then it is cheaper than the stx. both can switch between headphones and speakers in software.
i sound like a salesman... personally i'd love to see asus come out with an st/stx refresh with their new pcie chipset. all they'd have to do is replace the cmi 8788 chip with the cmi 8888, and they wouldn't even need to come out with a new product. it'd be even cheaper to produce because they can take out the PLX bridge chip