HP Dv6 is better in terms of CPU and GPU performance
The N55sf (assuming its the i5 variant with FullHD) is better in terms of multimedia and screen resolution

So it really depends on what your study purpose is.. it'll be nice to have a FullHD screen on your laptop, especially if you're working on graphic stuffs~
A strong CPU would also be good for faster rendering

As for warranty, it depends on the country you're going to... it'll be better if you search for feedbacks in the country you're heading to