Reactive interface question

By damage, you mean X% chance for the fire DoT, correct? Personally, I go for the DoT, because the -resistance is piddly at best, and not really useful except on hard targets. And on hard targets, you have a team (generally). And if you have a team, chances are there are other people with -resistance proc anyhow.

That would depend on the amount of DPS the user output, unless the DOT damage is much higher than I expect.

The questions that need answering for you to know:
What is the average DPS the DoT portion puts out at 50% and 75%?
What is the average resistance debuff you will have applied at 25%?
How much DPS do you output without the Reactive?

Add your DPS + the DPS of the 75% DOT.
Add your DPS + the DPS of the 50% DOT, multiply by (1 + the answer to question 2).

Which of those 2 is higher?

I am not sure anyone really knows the answer to the first two questions at this point in time. I have heard a great variety of inconsistent reports about the reactive proc. In a solo setting, activation times may play a factor in how many debuffs/DoTs you can keep active, further complicating the question. All this of course ignores the enemy resistance levels, since the DoT will be reduced vs. enemies with Fire resistance but improved relative to the enemy resistance vs. your primary damage type.

I'm pretty sure there is a misunderstanding here, but I'm not sure if it's yours about how interface works, or mine about trying to understand what you're asking.

Anyhow, please read Dispari's guide here about interface. It has lots of numbers, and tells you exactly how much each interface debuffs/damages the target.

The post you actually want occcurs much later in the thread: here. The -resistance debuff is 2.5% -res for 8 seconds. Assuming Dispari's understanding is correct, at 75% damage you will on average get an extra .75*5*[damage] and at 50% you'll get an extra .5*5*[damage] per attack. So against +0s that's 50.18 damage versus 33.45 damage, or an extra 16.7 damage per attack.

The easiest way to see which one is better is to ask, "How much damage to I need to deal to get the same amount of damage as the proc from the -res debuff).

In order to make up this damage from the -resistance you would need to deal 16.7/.025 = 668 damage in 8 seconds. Except of course that the resistance will only trigger every 4th attack, so really you need to deal 668*4=2,672 damage every 8 seconds in order to offset the dps lost (That's 4 attacks that tick for 16.7 damage less). So if you can deal more the 2,672 damage in 8 seconds, then the -res is better.

Now things are a little more complicated than this because of the limits in stacking of the debuffs. And so if your dps procs saturate, then the -res becomes more valuable. But that at least gives you a starting place, and would make me inclined to say that solo, the additional chance for damage is almost certainly better.