Basically, each boss has a level number.
The highest is the level, the most difficult is the mission and normally, the highest is the reward.
For my humble point of wiew (
http://forum.eredan.com/viewtopic.php?id=55126 ), each boss drop the playable rare cards at the same drop.
You can have more luck or less luck, but globally it's the same.
For sure is the type of rare :
Inferno gives only Nehantists playable rare cards
Xzia gives only Kotoba playable rare cards
Big Dragoon gives only Noz playable rare cards
Abyssien shadow gives only Zil playable rare cards
Big Deer gives only Sap playable rare cards
Big beetle gives only Nomads playable rare cards
The others bosses gives rares from any kind.
Note that the Hares mission gives more Runes than others bosses and the boss of the Zahal's treasure quest gives more Act 1 cards than others cards.
