MIT or CalTech?

Oh, that's a tough choice! Both are top ranked, extremely well respected in their fields. I really think you can't go wrong with either one. That said, MIT has a bit more prestige than Cal Tech, especially outside the US. But not by so much that, if you loved Cal Tech, you wouldn't go to Cal Tech.

Have you gotten into both? If not, then apply to both, see where you get in, see what aid they offer you, and only then decide.

If you've gotten into both, consider which one fits you best, personally. Because you can't go wrong - you have to pick based on fit. You can also let their financial aid offers influence you, if you're torn between the two.