I guess it comes down to personal preference. The RoG laptop is twice as heavy, but has a bigger screen. Using a 17.3" screen myself, I have to say that I can't see myself being able to use a 1440p or 2160p (4K) 15.4" screen, nor can I see a GTX 970M running games at that resolution well.
So if you really need it to be portable, go for the MSI one with the lower resolution screen, if you're going to keep it at home mostly go for the RoG laptop.