Most of your teaching will be centralised bar your supervision sessions, which are held by the Law subject tutors of your residential college. I'd have a browse of all the colleges and choose the college you want to be spending your next 3 years rather than going for XYZ college since it is the best at law. If you are keen about going to the top college for law, check the biographies and the concentrations of the law tutors of each college, then apply to the college with the tutors you like. IMHO, your academic experience at Cambridge will not be hindered by any means if you ended at up at Christ's rather than Downing (just an example).
Also, I would steer clear of all the "numbers game" - the whole statistical analysis of trying to get into Oxbridge "the most strategic way." If you're a solid candidate, this should not be a worry. I've heard this strategy being discussed every year when it comes to Oxbridge applications. I advise this out of personal observation. Last year, an acquaintance attempted this "numbers game" for Cambridge and thought he applied to the least competitive college after all of his statistical analysis (he admitted spending hours and hours on this) but ended up getting rejected. Funnily, another chap who was probably not as bright as the aforementioned acquaintance gave his best shot and went for a very distinguished college that he liked and was accepted.
Moral of the story, just focus all your energy in preparing for the academics not the statistics. Best of luck!