As a rule of thumb I'd look at what your build is good at and bad at, for example if you have a lot of inherent CritD from your buffs and other equipment it might be better to go for Dmg mods instead while if you got low CritD inherently go for that.
Obviously this is massively simplified picture of things and for the absolute best you need to do more research but it works for a rule of thumb.
so for a rom tac with 35% crit chance, I should use weapons with crtdx3/4?