Interesting topic even though m almost double the age now.
One of the first things to help decide is whether the aspiring candidate shall remain in Pakistan for university or go abroad. If one is going, O & A Levels have a slighter edge in terms of grades.
Second thing depends on one's memory level for Matric/FSc. and medical school and the other is if he can apprehend the basics and understand, O & A Levels is for him. The subjects in Fsc. are surely double those required for A Levels but then, A Levels require you to have vast in-depth knowledge on the subject. It is this art of understanding rather than rattofication that helps you once in uni.
As regards conversion, local system students face degrading to O & A levels in case they need to go abroad while the opposite happens in case O & A guys intend to study in universities in Pakistan. Back in the early 2000s, it was a ten percent difference. Not sure about now. This is done to encourage Matric, Fsc and studying locally. And yes, make sure to get your degrees converted in time and attestation done as well which means right as soon as you get the higher degree.
For those ov you in A Levels, my recommendation would be to give AS first and then A2 particularly students of mathematics and sciences because then the exams come, you will be having, in case of maths, four books to revise at once, and for sciences, a practical of A1 in the morning and theory ov A2 in the afternoon. The opposite happens for the next two papers. It is also correct that A Level exams are the most important in your life because it is these that determine how good a university you can be eligible for and graduating schools do have an impact on your professional career once you start. Hence, try to party a little lesser and study well. It's just a matter ov two years after all. Good luck!