What is the better filter steepness for resampling from 24 kHz to 48 kHz?
Lower steepness makes the range of frequencies at the upper boundary quieter. At the same time, iZotope RX 9 manual does not recommend higher values:
Higher filter steepness means better frequency performance of the filter: wider passband retains more useful signal, while stronger stopband attenuation provides better rejection of aliasing. At the same time, higher steepness of the frequency response requires a longer filter, which produces more ringing in time domain and energy smearing near the cutoff frequency.