Both should have...
Yen's hair is all right. Triss hair might be a little bit wavy, but she doesn't have curls in the game.Hmm... Do you count Triss' and Yen's hair as wavy? It's not super curly Afro-style, but it's definitely not straight. Well, Triss' is wavy.
Triss' hair does seem a little too vibrant red, fake, even (though admittedly she is a sorceress.) Less Jessica Rabbit, more Sansa Stark would be nice to see.
It's one picture/angle of her. We've seen much more of Triss, especially the close-ups. I'd say wait for more Yennefer before judging.
And in my own opinion, I still find her more attractive than Triss. There's something about Triss's face that puts me off. While she's not my thing, I don't blame the Triss fans for finding her absolutely gorgeous.