Interesting in fact, but Eric's answer is the best one : each kernel contains specific components offering (or not) benefits. All depends on your final use.
Get informations and rather try to compare different OS kernels (all OS are based on a microcoded core, even the ones driving your recent VCR or camera !). But perhaps, the last Sharp camcorder's kernel would be unusable for some PLC design project (to be verified).
Some OS ideas ? : OS-9, VRTX, VxWorks, QNX, Unix, Linux, Bill's Win XYZ or Embedded Win ZYX...
Be curious but careful : you could get lost in philosophical observations (what means "PLC" ? what means "real-time" ? to be or not to be ? and many other abyssal questions)
In a hurry ? Pick out brand names on this site, and call their local offices !
Regards