The sensors in both cameras have the same size but the samsug has higher pixel density which make noise harder to reduce, but that is still possible to do it better. So nobody can't answer the question until real comparison tests are carried out.
Judging cameras that have not been reviewed yet is risky business, so I may turn out to be dead wrong here. But the NX-300's higher MP sensor (20MP vs 16MP on NEX) will not affect video at all as Full HD is only 2 MP and the upcoming 4K standard is only 8MP. There is also no reason to think the NX-300 will any better in low light, maybe not even as good as the NEX, again due the higher pixel density. Then again, maybe that monster CPU will do a better job of in-camera (JPEG only) noise reduction. I use RAW so it doesn't matter to me, just give me the bits direct from the sensor and I'm happy. Regardless, no one has had their hands on a NX300 in anything like low light yet, so we'll see.
This camera's main claim to fame seems to be the 3D lens that will be available for it, its processing / shooting speed, and the fact that it's basically an android smartphone without the phone and all that entails with regard to apps. So if you're into shooting 3D through a single lens, you're golden. Gimmick, anyone?
Maybe the most important thing is that you can get the NEX-6 now, but you can't get the NX300 until March .. .or maybe June, you never know. The estimated price for Europe translates to around $1000 and I believe that is for the body only, so it also a step up in price.