Resist Modifier vs. Resist Debuff
 

I'm not sure how to word this question exactly, but do these essentially behave equally? For instance, will using a spell with resist mod -10 and no debuff resist as often as resist mod 0 and -10 debuff?

No.

The resist modifier is taken into account when the spell is cast versus a spell with a resist debuff which has to land in order for the effect to take place. A resist debuff's effect will affect future spells being cast on the target.

All things being equal and no other factors making a difference, a spell with a -10 resist mod will land more often than a spell with a 0 resist mod. The spells effect does not matter.

Example, Malo does not gain a greater success chance at landing because it reduces resistances. Its effect will only help future spells landing on the target once its resist debuff effect is applied.

Sorry, i meant if npc already had -10 debuff and then you cast a spell with 0 modifier is that the same as casting a spell with -10 modifier on npc that is not debuffed.

In that sense, then yes.