I think the aquamarine drops are great! However, like the others, I say go for the diamond studs. I think most people are saying go for the diamond earrings because they are a classic that will always be in style - a solid choice to mark this important event in your life. The other two items, while nice, only have a limited lifespan.
I have some diamond drops that are a quarter carat each, and they are pretty small, however they are beautiful and delicate looking. But I don't think I would go smaller than that.
Tiffany has some other beautiful classic jewelry items as well, if you decide you want something from this iconic store to celebrate your graduation.