I know heavy and special only drop on normal crota. Armor drops on both but I think hard mode is different than hard mode was on the vault. On the vault hard mode incorporated normal mode drops as an expanded loot table with both loot tables in each drop (double drops workout doing normal) Crotas End seems to not incorporate normal drops with hard drops. Hard loot being primary weapon and or materials, and armor and or materials. Because of this you can eliminate the primary weapon dropping by doing normal first and have an increased chance of getting armor simply by eliminating the weapon as a potential drop for your first roll, then doing it again with another chance for either an armor piece or a weapon. In addition to this it seems like the system intentionally avoids giving you duplicate drops in a single reward cycle. And on crota you can only get special and heavy weapon on normal due to hard being primary only. In the end for say gloves for example if you are looking for a better perk roll that fits your play style you can get up to 2 by doing normal then hard as opposed to hard first.