When to go what build?

Path #1

You should go Path #1 when looking for a lot of damage with your spells and AOE damage.
You can still burst their ADC or APC, sometimes you dont even need your death mark to pop.
Don't forget to activate your Hydra because it deals another like 300 damage in close range.

Path #2

Go Path #2 when you want to 1v1 a lot(invading), or if you want to splitpush.
It still works really well in teamfights, as long as you dont get stunned.

Trinity Force???

Trinity Force is a very good items for zed as far as i know:
Zed has a total of 113 attack damage on level 18, which means you will deal another 226 more phisical on your first basic attack after ulting(or any ability, every 2 seconds).
so also another 113 phisical damage on your death mark at level 18.