It is very hard to answer which one of these is better. If you want to do econ grad school you essentially need to double major in both of these. Applied math probably opens more doors overall and minoring in econ/finance will likely give you all of the options that an econ major would give.
MBA and Law school both care about GPA (especially law school). Math classes tend to be harder to get good grades in, so it could actually hurt your chances there to major in math, but if you do just as well, then you get some credit for it.
It sounds like you are early, so I would act as if you are majoring in both and drop one at some point if it doesn't seem to be working for you.