Is there any difference between software/hardware of iPhone 6 devices in different regions/carriers or all iPhone 6 devices around the world are exactly same from applications perspective ?
There are different hardware models by country / carrier to support different LTE bands. Apple has a list at http://www.apple.com/iphone/LTE/
In addition, on devices bought in China FaceTime Audio is disabled, while FaceTime Video is available. See http://www.zdnet.com/article/facetime-audio-disabled-in-china/