You may get more than from a regular beast. Bosses are larger and upon harvesting, usually, give more resources in their selected loot pool. For expediency though, I would safely estimate you will receive less overall Chitin if you rely on bosses. In the time it takes to slay and harvest the boss, you could have quite easily slain plenty of lesser beasts, harvested their Chitin, and could even be on the way home.