I’m not sure you’re asking the right questions as you weigh your options between the two schools. They are offering very different experiences. Of course, tons of students leave Lawrenceville and go onto medical careers. I don’t know how much your magnet school is a feeder for medical school, so you’ll have to evaluate that. Regarding swimming: there are ways to do club swim and lville swim team, but you’ll need to talk to the lville about what you want to do and how to balance that.
Regarding rigor: Lawrenceville is a very rigorous school and it requires a large time commitment, day students included. Kids often reflect that they cannot tell a day student from a boarder because the day students spend so much time on campus. This enables them to be fully invested in the community (they can take all their meals on campus if they want). You’re welcome to DM me about specific questions if you would like.