What college should I choose to apply to?

What college you attend at Oxford or Cambridge isn't that important - whichever college, you are sure to make friends, have a good time and receive quality teaching. If you apply to Oxford you will usually be interviewed at two or more colleges, whereas at Cambridge you will only be interviewed by one. (If you do very well interviewing at a college with a lot of applicants you may go into the "winter pool" where colleges who have less applicants pick extra candidates to make offers to.) You can always submit an "open" application to any college.
I would advise against applying to the very famous colleges like Trinity or Kings' at Cambridge as they get more applications than any others. Some factors to consider are where your department would be, and colleges that are near the department, or whether you would be willing to attend colleges further away. Additionally, the prospectus should have information on how many places each college offers for each subject - and not every college offers every subject. For instance Churchill College at Cambridge is known for taking a high number of science students.