Do colleges prefer AP or IB?

I'm just a freshman in high school right now, but I will be applying to some prestigious schools in the future.

To help shape my high school plan, I'm wondering what is more impressive to schools (particularly Ivy Leagues): IB or AP classes/scores?

If you have the option of taking either AP or IB classes and are aiming for top colleges, it's best to aim for the IB diploma. Earning an IB diploma is considered slightly more impressive than taking a scattered group of AP classes. Since the IB diploma is a program where you're required to demonstrate your skills across a wide range of subject areas, it's more concrete proof of your mastery of high school academics. If there are still some AP classes that interest you and fit into your schedule, you can always take those as well.

Keep in mind that selective schools really just want to see that you've taken a diverse sampling of the most difficult classes available to you. In general, APs and IBs are on roughly the same level in the eyes of colleges. That means that a student who takes AP classes at a school that only offers APs won't look any better or worse than a student who takes IBs at a school that only offers IBs. Even if your school offers both, a student who earns the IB diploma and a student who performs well in a balanced AP course load (classes in humanities, sciences, and math) will be on essentially equal footing.

I'd recommend reading this article for more information on the differences between AP and IB and which might be better for your specific goals.