The pooling system will even it out (in terms of getting into cambridge), so don't worry about whether one will give you a better chance of getting in than the other. That said, I might still go for Caius, as at least then you'll have less chance of ending up in a completely random college...if you think you're likely to be a very strong candidate (UMS above 95%, good at interviewing) maybe downing, though....at the end of the day, though, i don't think there is really that much difference between colleges.