(NOW THIS IS HIGHLY MY OPINION!)
The SX2 is the successor of the SX so it can do a few things more, especially Serato Flip which is awesome fun to work with (I am only scratching the surface myself) and it will do DVS.
Now of course when the SX2 will be release it is expected to get some good deals on the SX as an off sale. So if the new features (compare on Pioneer website) are not your cup of tea the SX looks good.
I do not particularly like the NV as it cannot do much more than an N4 and has screens.
I consider the integration of screens into a controller a waste. Even from the pics they are very small and actually looking down at a controller if you are in a DJ booth is something, when you are familiar with your controller, that is rarely done as you break eye contact with the audience and it looks not cool.
I always setup my laptop (slim MacBook Air or MacBook pro) the way that it does not block me from the audience but at an angle where I can glance at the screen without having to move my face away from the audience much, so I can stay in contact.
Also even at 13” and displaying everything on one screen, my laptop screen and resolution is WAY better than any included screens. If you are prone to what the industry called “Serato Face” then you will stare at the screens anyways, if they are in the controller or on your laptop does not matter.
I saw this actually when I moved from CDJs to controllers. I got response from some fans that I seem to be more connected to them as looking at the CDJ screens was much worse than having the laptop there, even with some CDJs on stands.