You should make your choice based on the structure of the course, the compulsory subjects, the optional subjects, and which of the two places your prefer, preferably after visiting both places or at least watching some videos about them.
It is no more or less difficult to obtain a place to read law at Oxford than it is at Cambridge. The two universities have similar admissions systems but there are some differences. For example, as you know, Cambridge tends to interview more candidates than Oxford does.