Trinity and Johns are great, but as has been said, they're all about knowing someone in Johns or Trinity who can get you a ticket. Plus, even if you can get a ticket, I personally would prefer to go somewhere less prestigous and go in a large group. Last year, most of my year at college went to Jesus (which is a very good ball in itself), but it was made extra special because we were all there together.
So in other words, maybe see where you're friends are going...