I had the NX and X-Fi, and what sounded the best was both of them together! Since the X-Fi is very limited on its I/O options, I went with the NX at first. The SQ of the NX is very good, but I really liked what the X-Fi Crystallizer did for my mp3's. So, I decided to run the output of the NX into the input of the X-Fi to be able to use the Crystallizer function (using the X-Fi in standalone mode, where it's not connected to the PC via USB at all). I had the X-Fi mounted on my console and used it as my main volume control. It sounded awesome.
One thing to consider though is to use the X-Fi in standalone mode, you have to supply it with 5 volts in some other way than from your pc's USB port. If the X-Fi is plugged in to a USB port, standalone mode does not work. I tapped into my cigarette lighter line with a 5v resistor and got everything all nice and neat and tucked away. It worked very well and sounded awesome.