Edit: I’ve been corrected, seems like Oxford and Cambridge do things more differently than I’ve been told by others. See below!
The Winter Pool (https://www.undergraduate.study.cam....ying/decisions scroll down) evens out applications so there aren't 'easier' and 'harder' Colleges to get into as such. Some Colleges consistently have a much higher ratio of applicants to places for certain subjects than others, but no College is uniformly harder to get into. Even where there are lots of applicants, they're not less likely to get into the university as a whole, because the pooling system evens that out.
As @island232 has said, choose the College you like most based on what it would be like to live there for three years. It will be your home and place of independent study for the time you are in Cambridge. However, Colleges are very similar and students who are pooled end up having a great time at the College which takes them, so don't worry about it too much!