I should first preface this by saying that I have only played the games and have not read the books (they are on order). I also chose Yennefer in the game. I'm also wondering why there are so many posts about Yennefer v. Triss on here and other forums as if people need to justify why they choose one or the other. In the interest of being mildly hypocritical, I am just throwing my hat in with the rest of you.
That being said, I felt Yennefer was a much deeper character. Let me explain what I mean: Triss came off as being more of a 'game character', that is, typically cute, friendly, and, for the most part, riding the coat tails of our much beloved protagonist, Geralt. This combined with her good looks and the assumed ginger obsession, probably seems like the more obvious choice for a lot of people, hence the great lengths people are going through in order to acquire additional content for her. I can understand this. But, and maybe this is just strange game mechanics, there are instances where Triss' character seems flat, one-dimensional and lacking depth due to her more 'game character' attributes. You don't really get a sense, at least in the third game, that there is a rich history between them even though there is, at least according to the games (which is what I am drawing references from).
There is also the observation made about Geralt losing his memory and that he would be able to choose his romantic interest anew. This would all make sense EXCEPT for the fact that Geralt fully regains his memory in the second game. We are never really told what that means and perhaps that is simply a deficiency in the narration.
Now before the hordes of Triss avatars descend on me, hear me out: I understand why you like her. I like her too. I just find that her character is just a little too typical and possessing too many 'game character' traits to keep me interested. There are hints of some complex situations that occurred, mostly in the second game, in their relationship but, for the most part, if you remember many of their interactions it came off as being more of a perpetual dating scenario rather than a meaningful relationship. Perhaps this is due to Triss obsessing over Geralt but her true interests lay at court and the scene on the docks where she is pursuing her 'dream job' and fleeing a real hell is only altered if you truly confess your feelings for her, then she just decides to stay. There should just be more tension in this scene, on the one hand she has this great job awaiting her and the promise of a better life and on the other hand she has the person she supposedly loves who is also trying to find and save a sister-like figure to Triss. Instead, the scene is rather flat, you either tell her you love her, or the more subtle "Stay with me" with some other strategic prompts to get her to stay. This scene should have had more depth, but because she comes off more as a 'game character', it is cut and dry.
As someone mentioned in this thread, Yennefer is a maze and Triss is more linear. I think that speaks volumes to my point.
Now, let's discuss Yennefer. I can see how people see her personality as being abrasive, complicated, and difficult. But she just comes off as a much deeper character. You can see, or maybe not see for some, that she has a troubled past, that she has some real issues outside of Geralt, that she has become cynical with the world, and that there is a much deeper history between the two of them rather than the 'perpetual dating scenario' that you find with Triss and Geralt. This can come off as bitchiness, but I didn't see it that way (and I also didn't read the books). It came off as being more real.
I fight with my wife all the time, we have both had difficult pasts and we are both very different people and perhaps because of this, I resonated with Yennefer far more. I can see that underlying their relationship, there is something more, something deeper, but that the vagaries of circumstance and the complexities present on the continent have forced both of them to adapt thereby making their relationship much more multifaceted than a cut and dry romance. There is history there and you can see it. There are two different personalities there and you can see it. There are two philosophical systems at play within each of them and you can see it. You can see what might be observed as 'contradictory forces', but often, as in literature, relationships of this nature become far deeper, and far richer because of it. So while on the surface, she can just seem like she is trying to control Geralt or give him a hard time, it's believable, and you can also see in other instances when the facade weakens and her true feelings come forth. All of this made me like her all the more because she seemed like a far more believable person than the alternatives.
I don't think there is any warranted criticism for people that pick Triss, but I also don't understand the shallow interpretations that people have of Yennefer. Maybe people prefer the simpler option of the two: one that just wants to be with you, comes off as being more friendly of the two, doesn't really give you a hard time, and is generally the 'cutsey, girl next door, damsel in distress' kind of vibe. I get it, I really do. But for me, and I think for several others (especially those on this thread, not to mention the people that have a broader sense of the total lore of the world and the protagonist in particular) can see the forest for the trees and others are only looking at surface level characteristics and behaviors without ever questioning why they might be there.
If you want to pick Triss just because she is more your type, then that makes sense to me. If you want to pick Triss just because 'you hate Yennefer and think she's a bitch', that seems rather silly, for there is far more than meets the eye.
I think that is enough for this wall of text.
Happy Trails.