1) I would consider RAM to be better than RCM
2) They're very different -
a) RAM+RCM are fantastic if you want to be a professional performer, Oxbridge if you want a more academic view into music and/or want to get a degree which is respected outside the world of music
b) People practise a hell of a lot at the conservatoires, not so true at Oxbridge...
c) Music college is full of famous (and not so famous) performers who may or may not teach you, Oxbridge full of famous (and not so famous) academics who equally may or may not teach you.
etc.
For me, I wanted a broader view of music and as I don't know whether or not I want to go into music I didn't even apply to music college, so Oxford ahem Oxbridge all the way. There are always postgrad conducting courses at the conservatoires if manage to get myself into that side of things. (PS see my sig...I am slightly biased!)