i'd think that brass would give u a clearer sound similar to a fretted note. as for graphite i haven't really played a bass with a graphite one before but i would expect it to be warmer and fuller sounding compared to a brass one. i might be wrong but that's what i hear.
if your frets are nickel, y don't u try putting some nickel ones on? like a zero fret so the diff in tone between an open and a fretted note is minimal.