If all you used was the 16mm pancake kit lens, the NEX will fit in your pocket. Put an 18-200 and you had better have really big pockets. Better pictures? After having shot both the D300 and the NEX-5, if I were forced to carry only 1 camera on vacation, the D300 would win hands down. For cycling/hiking where I am mostly outdoors with good light, the NEX wins. If I were to travel to a country where I will most likely never go back to, the D300 is the winner. I would not want to shoot with only 1 fisheye lens. I would want to use a 18-200. Put that on the NEX and it's small size no longer works as well.
Why carry around a huge, obsolete mirror box camera when you can carry something that will just about fit in your pocket and take better pictures?