I don't think it a good idea to approach the decision on the basis that one of the two universities might be easier to get into than the other one is. I suggest that you regard each of the two universities as equally difficult to get into, bearing in mind that the majority of those who apply for places do not receive offers.
If your academic potential is such that you could be a realistic contender for a place at Oxford or at Cambridge, then I suggest that you visit both and see what you think about the two towns, the colleges, and the university buildings. Please also look at the structure and content of the undergraduate law degrees at each university, and consider which course you would rather study. As you may already know, each university teaches the core subjects, but each offers a different number and range of optional subjects. Depending on which college a student is a member of, he or she may be taught in smaller groups at Oxford than at Cambridge, but this varies.
Some prefer Oxford because it's a larger (but not huge) city which has more non-university things going on. Some prefer Cambridge because it's smaller and has fewer non-university things going on. The look and feel of the two places you have to judge for yourself. If one college at either university appeals to you more than any other, why not try for that college, but please bear in mind that there's a reasonable chance that if you obtain an offer it will be at a college other than your target college (which won't matter, because you will still be going to Oxford or Cambridge, as the case may be, and "the best college" is usually the one that you are a member of).
Good luck!