A more expensive and presumably better antenna will provide increased "gain," which can make a significant difference in the amount of background hiss that will be heard when listening to signals that are less than strong. Also, higher gain correlates with sharper directionality, which can reduce multipath distortion, a common problem with FM reception The downside of that, though, is that the direction in which the antenna is aimed, relative to the direction of the particular station's antenna, may become more critical.
BTW, baluns for matching 300 ohm twinlead to 75 ohm F-connectors (or coaxial cable) are widely and inexpensively available, as are the simple 300 ohm wire dipole antennas that Herman referred to. There is undoubtedly nothing unique about the balun that was originally supplied with the NAD, and if you are in a strong signal area (for the stations you would want to listen to) purchasing a balun and a wire dipole antenna MIGHT get you satisfactory results for not much more than $10.
Regards,
-- Al