Liberty would have had the abortion, Manny the adoption

Honestly, I don't see a girl like Liberty having sex with a boy at 15 much less not taking it very seriously and getting every necessary precaution.

But, if they really felt the need to do two teen pregnancy storylines, it made a lot more sense for Manny to go through a pregnancy and eventually give the baby up for adoption. She had low self-esteem and was obsessed with finding someone that loved her. She was also impulsive for a longtime and never thought of long term consequences. She would have felt that a baby would love her unconditionally and make Craig stay with her before realizing towards the end of her pregnancy that she and Craig were unfit to be parents. A noticeable period of pregnancy would have also strengthened the whole "school slut" reputation that never seemed reasonable. Manny wouldn't have been the only girl in school that dressed revealing and most highschool students don't care that much about cheating between students out of their friend group. Everyone would have remembered the girl that got pregnant in the 9th grade. I know Ashley called them out but there was never any visual proof to validate it.

On the other hand, Liberty had a tendacy to be emotionally distant, analytical, and very goal-oriented. Sure she would still feel some desire to keep her unborn child/the fetus but she seems like someone that wouldn't hesitate to realize the long-term consequences and make the decision to terminate. It's more in line for her character to get an abortion and brazenly tell JT after she did it resulting in a fall out of their relationship. You can still do episodes of the fall out from JT's point of view and it brings up an interesting debate of pro-choice vs. father's rights.