Why would you use stock android while it has bloatwares and other stuffs that you cant do in CM.
CM11 has advantages over stock android,It has built in superuser and theme engine etc and smaller changes that wont be in stock android.If you have official kitkat for your device only then check it.You device might perform on that one better like battery and processes but CM supports most device officially too.I use and unofficial one but I havent had a problem.It is way more better than stock android..but mine was cheap android phone which had jelly bean and no official kitkat update and I needed to overclock it