HI shooting star,
thats a tough choice.. both have very different curriculum.. my kids are in chiltern house and must say it is almost the same as eton, they emphasize on confidence building, speech, mannerisms, and just being kids.. from what i know about eton and chiltern, they have a more wholesome approach to education..and they believe that studying is just by the books or memorising. they go all out to make learning fun.
on the other hand, montessori is famous for the certain form of structure. but be careful if you do decide on this as, i observe that, as montessori gets more and more 'famous' around here, the true essence of the programme gets lost along the way.. you have to really sit in and observe for yourself. heard quite a few harrowing tales about some so called centres that claim to be montessori but really nothing at all..
really depends on what you want for you kid. classes are smaller in eton too. you might want to consider chiltern as well. i must say im really truly satisfied with how my kids turn out. they are a lot more confident now and they truly enjoy school. eton is about $900 per month, payable per semester.