Yennefer is more canon pick (Geralts true love in books), Triss is more game pick (she was his love in game). Putting aside books and games... You can bang Yen more often xD, her story is better, her character is better and she looks so damn good in game

And of course you can do both but sth funny will happen near end of main game ;p

Yennefer, you have the option but Yennefer is the "right" option. plus, her "I love you" scene is magical.

Triss is my pick, but Yenn is the one an author would pick because opposites attract and drama.

If you don't pick Yennefer, you are not my friend anymore.

I prefer Triss. The way I see it, the books and the games are two different beasts, so what is considered 'canon' doesn't really apply in this case. I have completed the game two times, the first time I picked Yen and the second time Triss. Sure Yennefer has her moments, but to me Triss seemed more natural and a MUCH better person. Also, she is a redhead so that's a big plus for me :)  (I have more reasons than that, but I don't want to potentially spoil anything)

Yennifer is the wifey. Triss is the girlfriend with her whole life ahead of her.

Yennefer is supposed to be the "right" one story-wise, but she annoys the living crap out of me in ways I cant quite put into words so Triss is always my choice.

Plus redheaded girls are just superior beings, closer to perfection than any other. 

You can bang them? Why would you want to. I broke up with both during the game, good riddance.

They are both terrible people in their own way. Sorceresses tend to use Witchers in the books and series. But at least Yen would litteraly give up anything for Geralt and Siri. Yen is the type of woman that acts tough and bitch on the outside but is insecure and (somewhat) lovable on the inside. Triss is a bitch towards Yen though if you choose Yen. Besides Yen has a way dirtier mind than Geralt so after two playthroughs I choose team Geralt (fuck everyone) or team Yen.