We've not examined ESS(have anyone got such experience?) but we sold a hundreds of Plcm series ethernet devices in Russia before ESS even was announced. In one word, plcm are stable, all known bugs are fixed(last bug wasregistered and fixed in march). Plcm-e3 have STABLE USB, while afair SS use FTDI chip and drivers, it hangs up after even a little interference, f.e. when you powers up a spindle.
There are some difference in pulsing engine - PLCM provide 100khz, but it's enough for most purposes. And you can't do threading on plcm, bur I'm not sure that SS supports this function.