I had a DNX7120 in my last S4. I quickly sold it and upgraded to the DNX9980HD for the bluetooth capability and upgraded features (pandora control being a huge one). I prefer the functionality of the navigation and touchscreen, as well as the number of things that it is capable of compared to the factory RNSE. I have the RNSE in my 07 S4 and it works, but I can't use my phone with it, I don't have pandora or access to my ipod...the nav is slightly cumbersome to use, having a rotary dial for address selection...in my opinion go with the Kenwood. To add a backup cam to the kenwood you're only looking at $120 or so (just the price of the camera) whereas with a factory nav unit you're looking in excess of $600.
The only reason I haven't swapped out my factory RNSE for my Kenwood (that's just been sitting in my shop for the past 8 months since my last S4 got totaled) is the Sirius satellite radio subscription...which runs out in 2 months.