In a way, I think it was both; he was only really, at heart, interested in what she did for him, but what that "thing" was was to give him a feel at what legitimate, selfless love would be like (even if he's undercutting the ideal by using her to try and find it). Ziyal was the one thing untouched by the corruption and ugliness that had taken over and defined Dukat at least since he began presiding over the occupation. Having spent her adolescence in a Breen mine, she was cut off from knowledge of who he really was, and so longed for him that she was able to respond to her father only with love (indeed, as we know her failing was that she refused to see just what sort of a man he was). Even though she was clearly able to see he was considering killing her that first meeting, she refused to see him, only the ideal of him. I think that really touched something in Dukat, whatever last shred of selflessness there was in him, which is why he refused to go through with it and took her in. Taking Ziyal home and risking disgrace was perhaps the one thing he ever did that was truly right,
assuming we're thinking "right" as purity of purpose and not a police-state-Cardassian-style "right"
I think though, and here's the important distinction, that Dukat was too far gone to actually feel selfless, pure love. He could, however, still grasp the idea of it, and wanted a taste of it. I think Ziyal was his little fantasy he was playing. It was a reach for selfless love, for pure love, but not an actual example of it, because Dukat is simply past that now. And the tragedy or irony is that he used and manipulated Ziyal as part of his goal of creating this ideal of "pure love". With Dukat, it always turns to fantasy rather than his action or change or responsibility. He wants the Bajorans to love him. He wants Kira to respect him. He wants Ziyal and he to have a pure, perfect relationship, father-daughter. Ultimately, rather than do anything to truly earn these things, he just deludes himself into thinking he's doing things to get them, and this of course pushes those things, far, far away, and makes him all the more disgusting a man.
I think it's worth noting Dukat seemed to forget about his other 7 children and his wife, and focused on Ziyal- even after reclaiming Cardassia and vowing that "everything I lost I will regain". They were lost to him, tainted by who he is, whereas Ziyal had not been, so he could use her to play out his fantasy and reach for that ideal, which he longed for but was simply too selfish and, well, evil, a person to ever actually get.
Well, that's what I think.