I was having trouble deciding between Thunderlord's Decree and Fervor of Battle for the keystone mastery on Jax. Jax relies heavily on auto attacks, so I thought Fervor would be good, but Thunderlord's does a LOT of damage, but only after 3 attacks/spells. The problem here is that I don't know which one does more average damage per attack. Could someone help please?
Thunderlord's has an internal cooldown of 20 seconds, making it not so great for DPS based jobs.
Thunderlord's for midgame burst, Fervor for lategame DPS.
Fervor of Battle is better when you are able to force extended trades and for late-game DPS.
Thunderlord's Decree is better for immediate burst damage and in-lane harass.
However, I would recommend using Thunderlord's Decree on Jax pretty much always. He can apply it immediately with AA-W-AA for quick trades, which will be a massive amount of damage when combined with your ultimate and jump.
Jax's current build (Rageblade + Gunblade) emphasises AD and AP, so the scaling on Thunderlord's Decree and the hybrid penetration from Precision is more useful than the Percentage Penetration in the Ferocity Tree by a significant margin.
This may change next patch through, where they are adjusting several keystone masteries (and possibly the masteries below them).
Right now Thunderlords is the best option, next patch though you should probably consider fervor.
Fervor of battle
i don't play jax at all. when in doubt pick thunderlords
thunderlords is a safer pick escpecially since alot of champs can kite jax easily, particularly in the early mid game.
Thunderload's its op with ultimate and the abilities.
Thunderload's help to go 
lvl 6
[slayer-pantheon-thumbs]
