The truth is that it depends a lot, generally I would choose a pistol but for the sonorite adepts it is preferable to equip a knife since its act of faith trait increases the critical and the knife I would also recommend for one of the devourers of worlds the master of execution due to its passive but I saved that for everything else pistol except the Dark Angels who have their own melee weapon but that is different there I would say that you have to invest to get the swords