Processor, RAM and GPU performance is pretty much going to be a wash, with no real performance differences. The two advantages the GT 420M has are the ability to use DirectX 11 and Intel's Wireless Display and NVidia's Optimus for better battery life. The XPS has the better HDD (faster) and Wireless card (more robust). Battery life will probably be pretty similar, maybe with the XPS being a little longer. The XPS also an Aluminum case, making it sturdier and more durable.
Is it really matters, not to take extended warranty on dell (2year instead of 1) spending 5000 RS. Extra.
It depends on the person. Also, not sure if Dell still does this, but they used to give you the option of extending the warranty when the year of support was coming close to an end. Might be something to ask them about.