Let's go through the stats here...
Phantom Dancer: 50% attack speed 35% crit chance 5% movement speed -No unit collision
Stattik Shiv: 40% attack speed 20% crit chance 6% movement speed -Deals 100 extra magic dmg to several people at 100 charges
Personally, I grew a habit of building stattik instead of phantom dancer. What do you guys think? Depends on ADC or is one item almost always better?
is a mid-game item, 
is a late-game item. Until you got your second AD item, SS will beat PD. If you want to win fights before that, get SS. Or if you really need waveclear. In every other scenario, PD.
phantom dancer is better on AA heavy champs 
shiv if AS isn't that important 
I prefer Phantom Dancer for when I'm ahead.
Going way over the top to make my hyper carry champ be deadly late game is always nice.
Shiv is good for either champs that lack waveclear, or Lucian. PD, after the changes, is better.
Shiv vs PD? Batman wins.
I prefer Phantom Dancer personally. it depends on more than just the ADC. If my team lacks wave clear, I'll buy a shiv. If I wont be able to team fight until my 3rd item I'll buy PD. If I'm on an all AD team I'll grab shiv. ect.
used to be a lot stronger, but then they buffed to high hell. I never even liked as an item, so even before the buffs I used just because of some comment I heard in a "pro" LoL youtuber about it being better.
