yeah knights are bleh. it's kind of funny how addictive to play it is, but completing the 6 knight bonus can be so aggravating since you need both sej and kayle (yes, you can use a spatula + chain vest to make a knight but you seriously gonna blow two items to do that?). and yeah, it makes your champs borderline immune to autoattacks from a good chunk of champs but 6x assassin and item-stacked adc hyper carries like draven will still melt them, and of course you're super vulnerable to magic dmg.
6 nobles is just so much stronger. not only does noble buff apply to the nobles, but to the ENTIRE team. nobles still have magic dmg weakness like knights but they can handle the assassin and adc hyper carry threat a lot better due to the armor/sustain. it's also easy to throw a braum on top in conjunction with the leona to get guardian buff to help counter physical dmg even more.