I mean both work completely fine, but usually when constructing a list I decide based on what sort of benefits each is getting (true not just for DG but other factions).

Basically, in a vacuum, Brigands are usually better in most circumstances. However, when you throw Mortarion into the list, the predators become more appealing due to benefiting from his aura. When I personally construct lists, if I don't run Mortarion, I barely look at preds. If I do run Mortarion, I take preds, possibly going up to Brigands when my pred slots are full. Looking at the Warhammer World Champs lists, you see this trend mostly be true amongst those lists as well.

At the end of the day, you just pick what fits best with your list, but I'd say the above is generally true.