I'm an oddball who has both so I can cater the most to the group I'm with. But that's expensive.

Fending will give healers a wider margin of error, but will also cut into your ability to DPS and generate threat (so you'll have to work harder). The additional Parry on a full Fending set will help you take less damage passively, but there are a lot of people who just don't think it's worth it, and will argue against it.

Slaying will help you hold threat more easily, and will also help you kill it faster. The same people who argue against Parry will argue for "mitigation by proxy," that is, killing it faster to reduce how many times it can hit you, but there isn't any hard evidence or mathematical studies from Heavensward content to prove that killing it faster means taking less damage (or even if the boost to tank DPS from Slaying sets is enough to offset the mitigation loss from the lost Parry).

When 3.0 first came out, to help with the hard DPS checks in A3S and A4S, tanks were encouraged to work with the bare minimum amount of HP they could manage to survive with, which meant picking as many Slaying pieces as possible (though the fabled "pentameld" is more valuable). But now that groups have caught up to the target iLvl for Savage, Fending tanks are able to clear the content.

Ultimately, it boils down to personal choice. As long as you can hold hate and survive, you're doing what you need to. I have both sets because I like to be able to adapt, because 3.2 is changing things in ways we don't yet know, and because it gives me more pieces to spend Mhachi Farthings on. Throwing on my Slaying set to offtank in SwO can also output some really nice numbers, which is helpful in situations where you have no choice (read: that Void Ark run where the other two tanks constantly fight for the boss).