Played many games as Darius with either Clever or Tri-Force and both items work for him. Only problem I have is idk when to build one of the other.
So in what situations or team comps should I build Tri-Force? A few ideas would be nice just to get a variety of how to build Darius better into games.
Probably Triforce vs non-tanks, and BC vs tanks.
only if you are already slam dunking people and want to extend your lead. It can be quite a great buy, but the 
movement passive is better plus the armor shred.
Really depends on enemy team comp.
If they don't have any real tanks, Triforce will be the better option. Otherwise Black Cleaver is the better option.
Only exception is if you get stupidly fed early then you can grab a Triforce.
If you have transcendence then building both is nice for the extra AD,I usually use ultimate hat and transcendence as my secondary runes on most AD bruisers because I build alot of CDR items.
Cleaver is better for the armor pen, all trinity really adds over cleaver is attack speed (which he doesn't need and sitting on a stinger doesn't help much) and sheen is which only helps him with his W, most of the time he'll just be stacking and ulting rather then going for sheen procs.
