The voltage rating difference you referred to is a different matter. And everything else being equal a rating of 500V is actually better than a rating of 250V, although as a practical matter it is unlikely to make any difference in a 120V application. The significance of the voltage rating is that when the fuse **should** blow if that rating is lower than the applied voltage the fuse might not open the circuit properly (i.e., it might continue to conduct current). Or it might rupture or explode.
Also, the only time a fuse will "see" those kinds of high voltages will be when a mains fuse (or a fuse that is in some other high voltage circuit) has blown or is in the process of blowing. Under normal operating conditions the voltage between the two terminals of a fuse will be a small fraction of a volt, regardless of the voltage that is applied to the circuit.
Regards,
-- Al