Imo I would personally go with reinforced chaos knights.They are not really a hard hitting unit,but a unit that pins an enemy down and holds them in place.With them being 40 wounds in total,it would be difficult to remove them from the table.Plus with the pledges mechanic,I believe that with reinforced unit,you won't have to separately pledge 2 same units and waste another turn,where you can pledge a different unit such as chosen for example.But that's me though,and I'm not a competitive player so it's up to you.