If you think you're likely to do well in an entrance test then I would apply to Oxford, because everyone who applies there is automatically given an entrance test for their subject (as far as I know). If you apply to Cambridge with less than stellar grades (although I think your grades sound very good) then you run the risk of being rejected immediately without having a chance to prove yourself, which would suck. If you can do well at interview and in the entrance test and your personal statement doesn't bring up any red flags then you should be able to get into either uni even with only decent grades, but going for Cambridge is an unnecessary risk to take in my opinion.