Specifically caitlyn.

I'm guessing armor is for late game and hp/level is for early game?

I'm also guessing hp/level would be better if you didn't know exactly what you are going against?

It's the opposite—HP/18 gives you very little mitigation early while the flat Armor is most useful at level 1 and scales down from there. Use Armor for laning since it helps in trades and makes your pots more effective. If you manage to get to around level 8-9 without ever having to go back every game, then HP/18 may be better for you, but I highly doubt that.

the armor will always be better for adcs, the main reason is because you will be trading against the other adc, unless you swap lanes to a 1v2 top

the extra armor will mitigate more damage than the little hp you get early on, creeps + other adc are physical damage