Contagion/ED damage over time question.

Just checking if i'm getting this correct, if not, can someone explain to me the mechanics of this?

So if i take Spell damage nodes, that buffs the Damage over time amount of damage to both Contagion and ED. Meaning, the number will be higher, but not the duration.

If i take Skill effect duration, the damage number will remain the same, but it will last longer.

Is this correct?

If so, what's more efficient? Making them last longer, or making the DoT higher? I already have Increased Duration in Contagion, but i don't think i want that gem linked to Essence Drain, seems like a waste when i can just, cast it again if the duration runs out. And there are better support gems for ED too..

So would Spell damage nodes perform better overall?

I'm not sure i want to take either for now, i'm just researching my options.

I got all the chaos damage nodes, and the skills are powerful enough for maps, so i don't need it right now, but there are some spell damage nodes as well as increased skill duration nodes along my path, so i'm just trying to figure out what to do if i need to buff essence drain a bit (besides properly linking it of course).

Thanks for your input!