I think you should probably focus on the one you enjoy the most! I doubt that one will be significantly stronger for a Law School admission than the other, and you'll do better in whatever is most interesting.
If that doesn't decide it for you, then I suggest Biology. The reasons are, they have a lot of PoliSci majors applying, someone with a background in Biology will stand out a bit.
Also, on a practical level, when you become a lawyer, a technical and scientific background may help you find employment more easily. And I think biology would let you specialize in a variety of areas, such as patent law perhaps focusing in biotechnology (I know that's a place where there is a lot of demand and good pay) or even help you with something like medical malpractice, because you'd better understand issues and science in general.
But again, it really would be best to focus on what really interests you and motivates you, just because...!