Though to be honest of the two of them, Len is the most sensible one to go for, cause he's not a psycho manipulative jackass.
Well, what's the most sensible choice?
There's the girl who never did anything that you didn't okay first, never got angry, and whose entire stint of manipulation involved her lying about being supportive, convincing you to "practice" with her, and having a good poker face.
And then there's the guy who gets annoyed when you tell him to stop kissing you, angry and accusatory when you tell him that he's hurting you, and doesn't stop when you tell him to, which could probably be considered rape if she felt like calling him on it.
The former seems like a reasonable girl who is capable of keeping calm and /might/ turn into a yandere of some degree, which, depending on just how yandere she might go, could be anything from a pretty healthy relationship to her being incredibly possessive of her girlfriend. The latter seems like an abusive boyfriend or, at least, the beginnings of an abusive boyfriend, which is always unhealthy.