my take, as an undegrad at an ivy league school:
certainly there is NOTHING at any ivy league school that compares to the tutorial system at oxbridge. classes here center (centre) around lectures (some with several hundreds of student, even) and seminars/tutorials, but the smallest per class that you can hope for (most likely it is an unpopular class, or is very dry/specific/has a bad reputation) is probably.... 4. popular "small classes" could number up until 16. i once took a seminar where the professor said "the max i go for this seminar is 16, with 16 you can still have a good discussion."
having said that, there are big names at oxbridge and ivy leagues, so if you want to work with a high powered professor, there are certainly opportunities at both places.
the stuff oxbridge offers that i wish i could have:
1. the supervisions/tutorials -- definitely a BIG plus
2. oxbridge is just so beautiful that nothing in the US could compare!!!
stuff i like about my own school/the american system:
1. definitely the whole liberal arts thing. i've taken classes from latin american politics to the history of US foreign policy, as well as organic chemistry, linear algebra and shakespeare -- and they all count towards graduation!
2. the pastoral care. i might be totally wrong here, but my school takes a lot of care in ensuring that we are healthy/not depressed/coping well.
3. the school being private and having a BIG endowment. one could feasibly get a grant to do anything. independent research, doing non-profit, studying abroad etc. there is so much money to do so many things!
i may have gone a bit off topic.... so as far as an employer/jobs go, i think if you have Cambridge/Oxford/Harvard/Yale to your name, regardless of which school, an employer will DEFINITELY at least take interest in your application. of course, if you didn't do anything exciting at school, or have bad grades, and generally did not make full use of your undergraduate years, then would it matter so much that you have a bachelor's from oxford? or harvard? the name will get your foot through the door, but how impressive you are will be apparent in what you've done/accomplished at university.
also, there is this thing about "the grass is greener on the other side." my american schoolmates are convinced that oxford and cambridge are about as difficult to get into as it could possibly get while i've heard that people across the pond regard schools like harvard as being at the pinnacle of all academic prestige.
my take is just that it's not often that you hear of students from top high schools in america going off to oxford and cambridge, but definitely a good high school sends several to ivy leagues each year, whereas even at prestigious english public schools, more go off to oxford and cambridge than american ivy leagues. i think this boils down to the fact that not many people actually apply to schools abroad. however, this trend is changing, though, and more and more americans are crossing over to the UK and vice versa
just my thoughts!