For Kha'Zix, which keystone is the better?

I like to play Kha'Zix at top lane, and sometimes at mid lane (if top lane or jungler is an AP) and I've tested three possible keystones on him: Thunderlord's Decree, Grasp of the Undying, and Deathfire Touch.

Thunderlord's Decree is easy to proc as Kha'Zix can just Q and auto-attack once if he has a stack of Unseen Threat (his passive) to proc it. It is good for trying to get all-ins in lane phase but also makes his later assassination attempts much better. Taking the Precision mastery works well with the fact that both his passive and Thunderlord's Decree itself deal magic damage.

On the other hand, Grasp of the Undying makes Kha'Zix's laning phase safer and better, allowing him to sustain even better on top of the sustain provided by his Void Spikes. Constantly procing Unseen Threat + Grasp of the Undying on the enemy makes for a good harassing tool. However, it essentially forces Kha'Zix into a tanky bruiser build to make the best possible use of Grasp of the Undying.

Then, there is also Deathfire Touch. Both Kha'Zix's passive and his Q actually proc the most powerful version of Deathfire Touch (single-targeted and non-AoE). Kha'Zix would have to constantly harass an enemy to make this mastery useful early, but it is actually pretty useful later in the game with a bit of bonus AD if he is forced into longer trades or if he has to trade with another champion immediately, as it has no cooldown when compared to Thunderlord's Decree.

Ultimately, though, I can't decide which mastery is the better overall. It seems like the choice of mastery depends on the situation, as Thunderlords give a better burst, Deathfire Touch gives a better DPS, and Deathfire Touch gives a better laning phase.