I must say that, even if I had a bad 1st impression of Yennefer (mostly because I played TW2 with Triss), the more I played with Yen as a romance choice, the more I liked her. It's a great character !
I finished my 1st playthrough of TW3 with Yen, i'll do another playthrough (in normal or difficult mode) with her but with Keira alive this time (my most important regret !) and finally i'll do a playthrough with Triss when changes will be done in order to compare.
So i'm not a pro Triss neither a pro Yen. I really like both characters and I can understand why you choose one of them because of the books, the games, your personnal feeling, etc.