Now I know Geralt is no angel - far from it, but the way Yen treats him, both in the books and in the Witcher 3, I see her as a complete control freak. Manipulative...Overbearing...Domineering...and at times, almost psychotic.
Toxic would describe the unhealthy relationship they have going, something noticed by Geralt`s friends/fellow Witchers.
Triss is overall sweet, passionate, loyal and an extrovert with lots of personal charm - she`s great for Geralt.
In the books when Triss and Yen first meet Ciri in their respective times, Triss is concerned right away about how Ciri is being treated properly and shows real warmth to the little girl from the get go. She helps her without any attitude.
Yen just acts her usual bitchy, cold self to her for a good while.
In "The Last Wish" Yen shows she thinks nothing of using mind control on Geralt and marching him into town to get into fights. She also blasts him with a spell when he`s fighting The Reavers, causing him (and her) to be captured and bound.
Here we see that cold ruthless side to her.
I`ve read the Yen "book fans" excuses on here like "Ah but she was an unloved hunchback" (before she was able to change her looks through sorcery) Ah this explains why she is such an unlikable bitch?. Oh please.
In "Times of Contempt" when Yen takes Geralt to the Mages meeting at Thanedd, Yen is in typical form when she puts Geralt down in front of a couple of Sorceress in a humiliating manner. Basically she treats him like a child the whole time.
Also, during the Thanedd coup, It`s Triss that finds Geralt who is dying to all intents and purposes, she saves Geralt with the help of Tissaia de Vries. This is never mentioned by the Yen book fans though when they like to try and make out Triss was a coward and disloyal in the books.
One could also wonder how Yenefer fucked up so much by taking Ciri into a complete nest of vipers at Thanedd, she certainly knew how scheming and dangerous they were...
But yeah as far as choosing between the two, if you like scathing Women telling you what to do, what to say and what to wear etc. while emasculating you in front of others then Yeniffer is going to be a solid choice. No wonder Geralt would do a runner every so often