I haven't read the books yet, so my view is based on the game series only. And I would chose neither.
Both characters are cold opportunists with ulterior motives and they just whine about everything. If I would choose from the Witcher 3 romance options, I would choose Shani. But from the entire series I would choose Abigail from Witcher 1.
In my opinion, Witcher 3 is the least interesting option in terms of romance encounters. I felt that the romance plots were like cut and pasted together with lack of relevance to the grand scheme of things. I find Witcher 1 as the most coherent release when it comes to romance scenarios.
This is reflected from Abigail's synopsis. Although her role was limited to the first chapter, and briefly afterwards, her interactions with Geralt along with the messy situation in the village plus the doubts and conspiracy theories around her involvement with these ominous events added depth and a multi-faceted romance plot where several considerations come together and, ultimately, impact Geralt's choices.
Although there were similar attempts in later releases to make romance plots this complex, I still find these parts incoherent. For example, the synopsis of Syanna seemed like one of those complex romance situations, but her plot twists felt irrelevant to each of her stages that made no sense to the overall romance and storyline.