User blog comment:Tails365/Phantom Dancer SV Statikk Shiv/@comment-6365971-20130504173745

Personally I prefer PD, mostly because it was my favorite item (in coolness) before Ravenous Hydra came out (my new favorite).

If I had to choose 1 to buy on an adc, I'd choose PD because of the no unit collision. Of course, it depends on who you are and who the enemy is, but it's incredibly helpful to run from mundo cleavers and other pokes though minions, and run to safety. That extra 1% MS you get on shiv, that can be bought on boots upgrades later on.

The only people I ever get Shiv on are:

Nocturne so I can clear waves instantly with my passive/Hydra/shiv late game and that way I push so fast that I'm practically backdooring.

Ezreal because his pushing power is relatively low (at least for me) since most of the time I don't have crits for days where I can just aa once and the minions gone, I usually have to hit twice and throw in my single target Q eveyrwhere. With the shiv I can clear at an acceptable pace, and I have my E to get out of situations instead of relying on unit collision to work against the enmy team.

SO to anwer your question "when is one better than the other", always get phantom dancer (the extra crit chance especially goes with the Infinity Edge making you a ultra late game killer) as your main item, and only get shiv if you want to push like a boss, or if you have no pushing power and you want to be able to push fast then run.