These two alloys are of similar chemical composition, differentiated by the strengthening mechanism, Alloy 400 being solid solution strengthened and K500 being precipitation strengthened.
Alloy K500 has a higher strength but Alloy 400 has a greater toughness. K500 would therefore be a better choice in higher strength or pressure environments. Alloy 400 has varying magnetic properties while K500 is consistently non-magnetic.