DFT or TLD Which Scales Better?

Depends on the champion and purpose.

If you have a lot of AoE and/or DoT effects and/or sustained damage (i.e. Brand, Swain, Cassiopeia, Malzahar), Deathfire Touch is generally a better option.

If you have mostly single-target spells and/or rely on bursting down targets or short trades (i.e. Kassadin, Lux, Syndra), Thunderlord's Decree is usually better.

Also, you have to consider whether percentage armor or magic penetration would be more useful than flat hybrid penetration or CDR.

It's not about what scales better but which one can your champion utilize properly.

If in doubt go Thunderlord. But there are some specific champs like Brand, who benefits more from Deathfire.

I think - and if I'm wrong please correct me - but DFT should be better on poke mages who whither you down slowly from distance.

didn't that Aqua Dragon guy make a super educational post about this topic awhile ago? I wonder if someone could find it

T-lords. I used to take DFT on Taliyah, but remember, she is a burst/utility mage. Her Q, E, can proc T-lords very easily and it makes the damage skyrocket! DFT doesnt really help her, because the E's explode too fast to proc even 2 ticks of DFT, untill the last rock. Same applies to the Q. T-lords level 1 Q usually can take 1/2-3/4 of their HP.

They're different things for different purposes.

If you can constantly apply DFT for long periods of time, it's better.

Not many champions can do this reliably.

Decree is better almost always because it has base damage.

Alternatively, if you need the offense tree, and can't make use of furor or bloodlust, go for it.

TLD is better on taliyah for da burst.