i think it depends on the individual and unless we were allowed to apply to both places we can never really determine which is easier.
Across all subjects, they have a similar number of applicants of similar quality (on average) for a similar number of places; they also have similar entrance procedures. Thus I'd say that overall they're about the same difficulty to get into.
However, it does vary between subjects. There are a few where there is a clear (albeit fairly small) difference.
Generally i have nocticed that more ppl have AAB offers from oxford whereas nearly all cam ppl have to get AAA
I don't think there is anyway that somebody can say for sure which one is hardest to get into. There are so many contributing factors that a definitive answer is simply not possible. It really does vary from subject to subject.
Why does it matter anyway? A very talented applicant may decide to go for a subject like Classics, which typically has one of the lowest applicants-per-place ratios, while an average applicant may go for a "typically competitive subject" and get in because the college he or she picked didn't get many applicants that year.
It's impossible to judge the quality of an applicant by how many people a generalised table of statistics say that he or she had to fight to get a position.
To be honest I dont think many applicants factor in which uni is most competitive for their subject (if both offer it), rather they just pick whichever institution they like best and somehow it all works out. I'd imagine it's very unlikely that, without changing anything from your application (including interview), a Cambridge tutor would accept someone an Oxford tutor wouldn't.
Personally, one of the real reasons that sent me to Oxford rather than Cambridge was this ridiculous pool system they have going on - shallow, yes, but then so am I.