Conquering is definitely better if you have a total war CB. If you don't though, the cost of claims, the politics of CBs and the requirements to make the AI surrender can get in the way, especially if the target is involved in other wars; this last one is a big issue if you play with Starnet AI for example, because the galaxy quickly becomes a mess of wars and mixed occupations. Raiding on the other hand starts giving pops a few months after you park your ships over their planets, regardless of what is happening at the formal diplomatic level. So I think there are situational arguments for it.
From a less minmax and more "how it feels to play" perspective, early on you're desperate for established colonies and infrastructure, so taking another empire's homeworld feels much better than just stealing their pops, but after a while it just gets tedious to have so many colonies and to keep acquiring new ones that are either sparsely populated (and will realistically never have a large population) or have a big population that needs to be stabilized and the colony that needs to be redesigned to make it somewhat efficient. Gradually feeding pops into your existing colonies can feel more satisfying (provided you have enough colonies to start with that it really is gradual; you don't want to funnel pops so fast that your ability to add districts/buildings gets overwhelmed).
None of this is a particularly good case for taking a locked civic instead of a single AP though. Despoilation CB really needs to be made powerful enough to justify the civic