What are the strengths and weaknesses? Which one is better for university preparation?

IB refers to a program (if you are going for the full diploma), APs are individual classes. To participate in the IB program, you need to be registered through an IB school. However, for any AP course, any student can take the exam and prepare for it in his/her own. Also AP is probably favoured by American universities.
Therefore, in my opinion, AP courses are much more flexible to take.
AP wins!