Fervor of Battle vs Thunderlord's Decree on Kindred

Well, as you said, it is hard to answer this without giving an opinion... But i'll try my best :

Considering you're a top/mid laner, Fervor of battle is a better option. you can keep your stacks on and poke the ennemy laner every 5s roless, which will give you overall higher damages compared to thunderlord.

If you want to try a Burst playstyle, you should go for thunderlord, but this is more of a jungler option, since you gives at max 4 or 5 hit on a gank.

The other thing to consider is the T5 Mastery. When you reach fervor of battle, you get the 7% armor penetration, which is huge, especially for a laner. The T5 on the thunderlord branch aren't quite as good as this one for an ad exclusive laner.

All these reasons are why you should keep going on fervor as a laner, and maybe switch to thunderlord in jungle.

PS/opinion : You shouldn't play her mid or top, that's not her role and there are more usefull champions on these posts !

I Will give an answer Based on how the meta and the Masteries work (29-02-2016)

Thunderlord's Decree:

Gives you a higher burst, but doesn't do much, if you are planning to have prolonged fights it won't help that much because you will most likely only proc it once in the whole fight, however, when you are ahead(Something that is easy to do on Kindred Jungle) it will give you a nice help to quickly kill your targets(Unless they are Tanks), and in lane may give you some more trading potential.

Fervor of Battle:

Allows you to have more damage the longer the fight goes on(Something that can happen a lot of times since Kindred isn't the best assassin) and if you go Devourer (or another High AS build) you will stack it fairly quickly in a fight and it might give you a lot more help than ThunderLord's since it sinergizes a bit better with Kindred's overall kit (Kite and high mobility).

Recommendation:

If you are gonna play her Jungle i would honestly take Strength of the Ages(In this meta) since it will give you more HP to stay healthy in fights and you can stack it fairly quickly, if not it truly depends on their's and your's TeamComp.

For instance, if you have a kite composition, Fervor will be of more use since you will stack it and deal extra damage in the fight.

If you have a Teamfight and Pick composition, Thunderlord's would be better since it will give you higher burst.

Final remarks:

I would say just take Strength of the Ages ( If you are gonna play jungle) because of the overall benefit it gives you, but if you lane with Kindred, it depends, are you gonna be looking at long and Drawn out fights in your lane or quick trades? Hope i was able to help