Ideally, you should have both, especially if you do a lot of Duty Finder content with random folks. As someone suggested, it's a good idea to start with Fending and to slowly work your way into Slaying. Using Slaying requires proper cooldown management and also puts more strain on your healers to a certain degree if they also DPS, as the difference between full Slaying and full Fending tends to be about 2 to 3 GCDs before the healer will need to heal--though every healer has their own view of how low the tank's health can get before they need to heal.
I'm not a serious tank and in HW thus far I've felt more comfortable using Fending (I used pentamelded accessories in ARR). As a healer, I prefer Fending tanks by default only because I don't know if the tank knows what they're doing (and I have to say, with the shift in the meta, there are a lot of tanks wearing Slaying accessories who have no idea that they need to do extra work to compensate for the lost health). But a good Slaying tank does fine if they have the left side gear and the skill necessary to pull things off.
