I think that one of the biggest differences is just the difference in looks between the two. If you are adventurous then the NX should be right for you. The RX is is much more on the conservative side with styling.
The NX is going to have far less horsepower than the RX. 200 hp compared to 295 hp respectively. The NX is also 5 inches shorter than the RX, so if space is what you are concerned with, the RX may be what to go with.