The biggest differences between them are the display and GPU. The N550JV has a better IPS touchscreen display which offers much better viewing angles, better colors, and ,of course, touchscreen capability if you care about that. The Y510P has a regular TN panel.
The N550JV has the DDR3 version of the 750M which will be worse than any of the Y510P's GPUs since they all use GDDR5. Other than that, the N550JV looks to have better battery life and is slightly thinner. However, the Y510P is cheaper for better performance. Personally, display quality is a priority for me so I would lean towards the Asus N550JV.
You can also get a Y510P with SLI 750M for $1000, but it seems microstuttering is a problem: