When to choose Deathfire over Thunderlords?

Q: Are you Jhin?

Yes: Deathfire No: Thunderlords

Burst: tlords Damage over time: deathfire

You want to lose?

Yes: Deathfire No: Thunderlords

somechamps have trouble hitting a champ 3 times in a row, ie , some like the percent magic pen and have a dot that makes death fire more effiecient

Your going to get various answers based on preference. If you want a pretty reliable method of finding the popular opinion on keystones I would use a site like na.op.gg to individually look up champions.

Not having single-target or persisting AoE spells like Anivia's ult is a big deal regarding Deathfire Touch. I would never take DFT on Ziggs for example.

One major problem with DFT that I've noticed, is that it's DoT can screw with your ability to tower siege, cause it will keep drawing the tower aggro to you.

But if you can play around that, it's pretty good. I actually use if a bit more often than thunderlords. I personally Prefer DFT in instances when I need to deal with a crowd, because it can effect multiple enemies without going to cooldown. (I use it on Just about any battle mage)