I personally have no stake in this, I am just observing from an outside perspective so I cannot really say why some groups whose webkit + V8 over JSC, you would really have to ask them.
I am guessing there are probably non-technical points (eas of commit access, architecture, roadmap etc)
http://www.arewefastyet.com/#machine=12&view=breakdown...
each beats the other on some benchmarks. Is there another reason for your preference?