depends on the build. I have used Ignore Resist and Weaken on different builds. the good thing about IR is that it only takes up 1 space on your build. Weaken, if you want it to work well, needs 2 spaces minimum to be on 100% all the time, a 90% Crystal Weaken and 30% Epic Weaken or the Myth Stone that does 30% Weaken. the other good thing about Weaken, is when it is over 100%, it starts causing negative resistance to the enemy, so extra damage. so IR is good for builds with limited space for everything, and Weaken is good for builds built around it that want the extra damage.